Sure, GitHub is sexier, the web interface is much slicker, trendy and all. On the other hand, Google Code is here to stay forever or so, I know SVN, the YOURLS project has all its ticket and source history there and, despite being a bit raw sometimes, the interface and the service get the job done.
One argument I’ve been given a couple of times is “you’d get more contribution on GitHub”, but I’m not convinced: why would it be easier on GitHub (just because you just click “Pull Request” and save the minor step of generating a diff file?). The counter argument that easily comes to my mind is: spending hours on learning new tools and migrating the project and its history (issues and commits).
But maybe I’m not seeing the big picture and there are factual reasons why hosting on GitHub would be more beneficial to the YOURLS project. If you believe that’s the case, please do comment and convince me :)