In the land of Elohaloth and in most of the lands that are anywhere near it, magic tends to consume a lot of energy. It helps a lot if the mage of witch knows the true name of the object or person she or he wants to cast a spell on, but even so, most lone spellcasters can only cast cantrips or at most, a spell powerful enough to create a small fireball. It was the Archmage Harkon who discovered, by accident when he was just an apprentice and called on a fellow apprentice to help him, that spellcasters could pool their energy. That was a thousand years ago.
Spells were able to grow in power and in force, but it was when it was discovered that non-magic users could lend their support to a spell, that the Great Spells of Cloaking,Blocking, Revealing and Healing could be used to improve or protect whole regions. The smaller spells continued to be used of course, but were rarely used on a large scale.
To keep one of these Great Spells going, a large number of people need to want to keep it going in their hearts. So unpopular rulers are not going to be able to persuade enough people to lend their support. Also, such spells can be fragile-in one case, a kiss from an adventurer was enough to break a sleep spell upon a deeply evil sorceress and plunge a whole duchy back into a horrible magical war.