Network effects. There's entire subcultures that have de facto clustered themselves onto GitHub, like the Erlang community. Most web developers, as well.
You're right the actual GitHub interface is not as amazing as it is often heralded. I've always thought the issue tracker was a joke, for instance.
The issue tracker is absolutely a joke, but being a standard for company codebases is a huge moat. If it works well enough, what company wants to spend time moving a codebase/history.
I speak this as someone who managed developers at a company that was still using CVS. And yes, we discussed moving to git every so often, but it always was lower on my priorities than new features.
You're right the actual GitHub interface is not as amazing as it is often heralded. I've always thought the issue tracker was a joke, for instance.