I can only speak for myself, but I love GitHub because it's easy accessible for me. When I was learning how to use git and code in general (well, I'm still in that process to be fair), it's great to have a visualitation of what you're actually doing. It's even better when that interface is well-designed (at least I consider GitHub to be great UX), so you don't have to learn using another UI on top of all the new stuff you already learn as well. I think that's a factor you underestimate when you've been working with revision control and code for a long time.

Additionally, the collaboration aspect of GitHub is great as well - As it's a big plattform, a lot of developers already have a GitHub account and it's very easy to contribute with forks and pull requests. Submitting patches via mailing lists isn't everyone's cup of tea.