I'd say the choice of Chameleon was driven by them wanting it to look "modern" or iOS-like. A native Git/Github client like this doesn't make sense on a device where you can't develop code (except in the web browser with Cloud9 or something).

In respect to the Windows question, I wouldn't be surprised to see them ship one in a few months, once they iron out the bugs and determine exactly which set of features a native client needs to provide.