Even when there are huge network effects like this you do occasionally see big shifts, usually when the incumbent screws up, or there’s a competitor that is so much better that people want to switch anyway. For example SMS->BBM->WhatsApp, MySpace->Facebook->WhatsApp, Digg->Reddit, and of course Sourceforge->GitHub.

So I guess the deeper question is “why does everybody else still use GitHub?” and the answer is:

1. Microsoft haven’t screwed GitHub up in any significant way (like e.g. Digg did).
2. There aren’t any competitors that are so obviously better than people want to use it despite the network effects.