I can say personally that the only reason I use GitHub and would never consider SourceForge is because of the D in DVCS. SourceForge always wanted to be your world: You have to give your repo over to them and let them manage it on their servers. By doing that you are now locked in to their world.
With git I don't care if GitHub goes down for a month or accidentally loses all my code or kicks me out and starts to charge for hosting. It's because they are hosting a copy of my repo and are not the sole keeper. I always have my repo with my precious source code history and it's impossible for them to take it away.
I suspect a lot of hackers are like myself and don't like giving up control where they don't have to.
That and GitHub just doesn't feel as hostile as SourceForge.
With git I don't care if GitHub goes down for a month or accidentally loses all my code or kicks me out and starts to charge for hosting. It's because they are hosting a copy of my repo and are not the sole keeper. I always have my repo with my precious source code history and it's impossible for them to take it away.
I suspect a lot of hackers are like myself and don't like giving up control where they don't have to.
That and GitHub just doesn't feel as hostile as SourceForge.