Enchantment spells are almost always Will saves, and don't affect constructs or undead. So, it's not as useful as, say, Transmutation. That's why, IMHO, it goes in the middle group. Also, the Hold spells are nice but Clerics do those better.
Divination is nice, but you keep running into the simple fact that Clerics can cast most of the same spells or better. Also, many of the divinations have material components that other classes can avoid. If you play with Psions, they get many of the spells sooner. To top it off, at a few levels you only get 1-2 choices (I'm not saying Discern Location isn't good at 8, but that's your only option, and Foresight at 9 just sucks)
Necromancy is just a tiny school. It's got a few great spells, but the key is FEW. Two 5th-level, 1 6th, 2 7th, and 2 8th-level spells in the PHB?
So there's the reason for the difference. Some categories have a small number of spells. Be a Transmuter and you have tons of options at each level, be a Necromancer and you're stuck picking specific ones. The power of each individual spell may be just fine, but flexibility is an asset.
In 2E I never saw many illusionists, but now in 3E I see quite a few. Some because they're Gnomes, some because of the Shadow spells.