I'm guessing because teams don't want to be limited to working on one project so much and would rather be able to quickly go to the next one.
I'm guessing that's why recently there's been more short anime being aired.
And some anime it's much easier to fit that 12-24 episode range.
Something like SAO can be split into definite arcs without causing problems to the next one (or I presume, I've only watched season 1).
Therefore, it can nicely fit into 24 episodes.