Mostly just that it's the platform I'm most familiar with and felt reasonably confident in the TAM.

Partly the challenge, too, though! If I were designing within the constraints of a Gnome WM, the result would probably look totally different from where this project is now. It's kind of fun having, on one hand, a lot of OS restrictions to work around, but on the other, native first-party multi-touch, Core Animation, and a rich history of platform conventions to draw from and adapt.