1-Evil Magic is more fun
Evil magic is thought by most writers of magical spells to be both much easier to make and much more fun. Which would you rather spend time making-a complex, lengthy spell to cure cancer, or a quick spell that creates purple lightning to fry your foes with? Therefore most known spells are evil in nature.
Most magic is made by the wizardly equivalent of the military industrial complex and is used for war, to kill and injure soldiers and destroy forts and military machines. The victims of such magic often think of it as evil even when it is no more evil than swords or musketry fire. Of course some such military magic is very evil indeed.
3-Good Magic is highly restricted
In many countries it is not just evil magic that is tightly controlled and clamped down on. Healing magic can put skilled doctors out of work. Magic that automates tools puts large numbers of workers out of work, and the workers then either turn to crime to survive, or have to be paid benefits by the government which would much rather spend the money on other things.
4-Only demons grant magic
The only way that mortals can get access to magic is by making deals with demons, and they are unlikely to grant any spells that are going to let their users go to the celestial realm after death and thus escape their clutches, as mortal souls are a currency in the Nine Hells. Therefore the spells that mortals have access to are evil.
5-Magic gets out of control
Any magic that is cast is like a fire in a dry forest and quickly gets out of control, and fills the mind of the caster with a huge wish for power, and this applies to any spell bigger than a cantrip. As a result most magic users end up twisted and evil and a little deranged.
6-Good magic is harder to cast
Learning *good* magic such as healing takes many years of study to stop it going random, whilst combat magic and *evil* magic is much easier to use. Therefore most magic is slightly or indeed completely evil.
7-Magic warps the user
Magic has a very good chance of leaving it's user warped to the point of gross insanity who is likely to go on a rampage and slaughter everyone nearby and even healing and other neutral and indeed good magic has the same effect, therefore few good people will dare to cast spells.
8-Good magic hurts the caster
When someone casts a good spell such as a spell to heal someone or even a neutral spell it drains his or her life force to power the spell and can have fatal or at least sickening results. Combat magic and evil magic on the other hand drains the needed power from the target of the spell and is therefore used more often.
9-Only one gender may cast good or neutral magic
Because a great archmage or archwitch was extremely sexist long ago an incredibly powerful spell was cast which means that one gender (the GM chooses) can only cast evil magic, and is therefore legally banned from casting spells. Perhaps the PCs are on a quest to break this spell.
10-Good magic was successfully suppressed
Because it was threatening the jobs of the elite and the humble worker alike and was of little value to the underworld , good and neutral magic was successfully brought to an end after several years of campaigning against it, and the exile, execution or jailing of it's users. Whilst there was a campaign against bad magic too, it was kept in being by the mafia who found it too useful to give up.
11-Good magic needs fancy stuff
To cast good or indeed neutral magic you need rare and expensive paraphanalia such as a six foot staff of rare pico-wood, the gems from a dragon's hoard or a royal sceptre whilst evil magic just needs the right knowledge , the power of speech, and waggling fingers. Legal magic is therefore very much restricted to the nobility.
12-Only the Order of X can do good magic
A certain magical order of mages has monopolised the casting of good and neutral magic in the area, and whilst they occasionally fight malignant mages and wicked witches for the good publicity, they come down very hard and sometimes fatally on those who threaten their magical monopoly. And they are not the sort of mages that you want to mess with.
13-Leave me alone please!
Once the news gets around that someone can cast good/neutral magic, they are badgered and hassled by beggars and average people alike begging for magical favours and gifts,most of who cannot afford to pay for it. Therefore most such people keep their magical skills to themselves. Evil magic users don't have that problem as most people are either too scared to bother them, don't want to, or fear legal trouble if found out.
14-Good mages get conscription
The local ruler has a like of conscription for any good and neutral mages that his forces can catch, and they are then conscripted into his projects for months or years on end, building, healing, or scrying for him for low pay. Bad magic users are much more willing to do whatever they need to do to avoid being rounded up.
15-Good magic causes bad luck
Good and to a lesser extent even neutral magic cases bad luck. Neutral magic just causes bad luck for the caster but good magic causes bad luck to anyone nearby as well and is therefore unwelcome at best and illegal at worst. Oddly, evil magic causes good luck for everyone nearby except the target of the spell.
16-Good magic is impossible
Good or neutral magic is nothing more than a myth. The only magic known to exist is that which is used for evil ends, to kill, wound, bewitch people or raise the dead. As such, most areas ban it outright for safety reasons.
17-The gods don't like magic and like good magic least of all
The gods and goddesses see magic and in particular neutral/good magic as a threat to themselves personally and react accordingly, with anything from a bad afterlife for the caster when he or she dies to instant death by lightning. They are more tolerant of evil magic, as the user is most likely going to hell after death even without their intervention.
18-The demons hate good magic
Demons hate good magic and see it as a threat to them and are not all that keen on neutral magic either. And it takes a powerful magic user to fend off one of the Greater Demons when they show up.
19-A good spell for one is a curse for another
If you use magic to heal someone's sight for example, someone a mile away goes blind. When a good spell is cast on someone, it harms someone else or causes them bad luck and the caster is not going to be popular to say the least. Many areas ban all use of magic because of this.
20-Good magic polymorths the user
Good magic polymorths the user into some other form,such as a fairy of some type. Evil magic also has the same effect but turns the user into an Orc or some similar creature. Only neutral magic has no such effect.
21-Good magic argues with the user
Magic is a living force, and it only casts itself when it wants to. It likes things such as combat magic, curses, and necromancy, but is bored by good or neutral tasks, and the caster has to beg and plead for the magic to do what he or she wants it to.
22-Good magic can only be cast in a holy place
Non evil magic can only be cast in a holy place that has not been profaned by evil. As such the owners of said place have extremely strict rules for who can cast and when and what spells can be cast there. Evil spells on the other hand can be cast almost everywhere as there is far more evil energy in the wworld than good energy.
23-Good magic is really hard to learn
Good magic is like advanced calculus -really hard to learn, and neutral magic is almost as hard to learn. In comparison evil magic is like learning basic secondary school mathematics and is therefore much more common.
24-Good magic warps the area into a Disney like world for miles around
Good magic warps the entire nearby area into a Walt Disney like world for miles around. Crowds of people suddenly find themselves breaking out into random songs and complicated dance routines against their will. Kisses suddenly make women pregnant or make them fall in love with the kisser. Military forts become huge multi towered palaces where the garrison is suddenly far too small to hold the walls successfully. Animals talk and brooms come to life and flood houses with buckets of unwanted water.
Queen Yocasta once took a small castle by having a wizard heal a blind man, causing a castle meant for a garrison of 150 men to turn into a giant royal palace ten miles in diameter. With the garrison now stretched really thin it was easy for her army to scale the walls. On top of that she had a ready made royal palace to rule from.
25-Good magic harms the one you love
Good magic causes bad things, like diseases, curses or bad luck, to happen to the person that the caster loves most, and as you can imagine few people will willingly cast it as a result unless they have to.
26-Good magic is like a love spell
If you cast good magic on someone the person falls headlong in love and lust with you, whilst neutral magic makes them want to date you, only evil magic not causing these effects.
27-Good magic costs more
Because it is harder to create in the first place, good or neutral magic costs much more to buy then most of the evil stuff. Sometimes much more.
28-Magic twists the caster's personality
Whilst evil spells turn the Mage or witch into a power hungry person who only cares about themselves, good magic makes them only care about the happiness of others. Most people don't like either fate and so are very careful to cast only the most neutral spells.
29-Good magic sends the user to heaven
Good magic causes the user to have his or her soul snatched up to the celestial realm, just as bad magic causes the soul to be snatched down to the infernal realm.Most people who wish to go on living in the mortal world only cast strictly neutral magic.
30-Good magic lowers your sex drive
Non-evil magic lowers the caster's sex drive for weeks at a time whilst bad magic raises it instead and there are those who use minor curses for others as a form of magical Viagra for themselves. Monks and nuns who feel tempted someuse good magic for the opposite effect.
Additional Ideas (1)
For every Good spell or other Good magic that is used, some form of new Evil is released or increased to the world. However, Neutral and/or Evil magic does not create something Good.
This can be weaponized by sending Good casters to an enemy land to secretly cast a lot of Good magic. This would cause a greater amount of Evil to be released in the local area possibly causing crop failure, plague, earth quakes, monster incursions, random curses, etc.