because where as evocation is energy (cold, fire and lightning) and such, conjuration is summoning, and acid is sort of loosely implied to be a substance that you summon forth, a nasty material that causes burns and owies. If you were summoning rocks, it'd make sense, right? What about a lot of tiny rocks, pebbles really? Powder, like Lye? Anyway you get me right? Yes, there's the whole elemental aspect: earth(acid) fire, water(ice, and even then some water/ice spells are conjuration too, when it's not about sheer cold), lightning(air) are the big 4 elements. Evocation is not the "elemental" school, just because it has so much fire and lightning and cold spells, its also force and other stuff. Anyway, of those spells are more your focus, take the Elemental wizard schools. Kind of limiting given how much the other schools allow for, but anyway
Basically because Acid is the one element type that's a physical substance rather than energy. In every other respect it tends to act as an element type. (Specific damage type that can be resisted and so on.)
It's one of those oddities of the game that's probably best not to think about too hard, really. Such as the tenuous association of acid with earth, since every other element has an easy-to associate damage type, leaving the castoffs to Earth, since Energy Resistance: Stone doesn't make much sense, and Energy Resistance: Bludgeoning would be very hard to balance otherwise.
