Also, Photoshop has a much higher market penetration than Illustrator. It's far easier to find tutorials and resources for PS, which serves to further increase its influence over the market. In short, people use Photoshop (or the Gimp) because other people use Photoshop.

I certainly would like to see more Illustrator content show up here, and if I had the time & energy I might do something about that myself. Alas, I haven't had the leisure for mapping in quite some time, and I don't really see that changing for a while.