Does anyone else notice how all the top ranked answers address the technical differences between GitHub and Gitorious? GitHub certainly has a great product, but that is not a sufficient condition for being popular (it may not even be necessary). I don't know how GitHub got a head start, but the answer to their continuing success is the network effect.
Just goes to show how there will almost always be different communities online with differing levels of focus.
StackOverflow's community clearly values the intricacies of tech.
If this conversation was happening in a LinkedIn group, I can imagine many more discussions on distribution models, market size, or even strategic partnerships.
HN has the unique blend of tech+business providing unique insights and answers that takes a lot of these different factors into consideration.
One of the things I love about the Ruby community is that it truly embraces change. Rubyists (and particularly Rails hackers) jumped on git, and since GitHub was built by our own, it was supported heavily by Rubyists. Engine Yard gave them free hosting for a long time, we blogged and told all of our friends, and like any social network, it just grew from there.
