Because I currently have no way of working with AROS. I have a PC, but it's not in a usable location. I use Macs primarily right now, and the last time I checked, the latest AROS didn't work either on PPC hardware directly, or on top of OS X. Either of which I'd be fine with.
I'll just develop against libSDL, and let others port it to other platforms. (I'm attempting to get the latest E-UAE running with diskfolders on my 10.3 machine, with no success right now...)