Also, if I want to setup a repo and put it on GitHub, it's stupidly, ridiculously easy. SF isn't hard, but it isn't as easy.
Open source software is about PROGRAMMERS collaborating. GitHub makes that incredibly easy and straightforward. Want to join a project? Fork the code and tweak til your hearts content. Your code shows up in the network and other people can find/clone/fork/hack til their hearts are content. Want to contribute to a SF project? Um... contact the dev and hope the project isn't dead?
SF is designed to be a kind of Download.com for FOSS. For that it's fine, but GitHub is designed for developers to share code.
Also, GitHub is great because it also does private code hosting, so if I'm working on my own stuff or FOSS stuff, I can use GitHub for both. That is incredibly convenient.