¿OSS compiler availability? AVR has GCC open source compiler readily available, PIC has the free hitech C-lite what compiles without optimizations(horrid code)
¿team members familiarity/expertise with AVR?
I don't know for certain, but these two seem like reasonable guesses. One thing about the AVR is that it has the GCC compiler for it, so just about any platform can be used to compile code for it (you could probably even use a souped up Amiga if you wanted to - with a bit of hacking, of course!). Whereas the PIC doesn't have this openness; it was one of the things that drove me from the BASIC Stamp to the Arduino (I could no longer compile my code under 64-bit). Plus, GCC support for the PIC line is limited for some reason (only a couple of lines from the family, from what I have gathered, and it doesn't seem to be updated often, either).
So this openness might have driven a decision (???)...
That, and the people behind it may have been more familiar with those tools than with the PIC and its toolchain.
Maybe the founders will chime into this thread, and point us to some historical documents or tell us in more detail why they did choose the AVR? All I know, though, is that I am glad they did!