I think it's because it's not seamlessly integrated into Github and instead relies on external services that aren't intercompatible. If Github integrated/bought one of these services and made everything work smoothly with their billing system, I think many people would love to contribute. Maybe even make "corporate" contributions, although I don't know how that would work.
I'd much rather have a 'corporate' version where you could sell commercial licenses through github for $250-$350 to bigcorps and VC funded startups than a gittip where you can beg for 5 cent donations from developers.
I would love this. Has this been discussed anywhere?