which could enhance the contribution rate from the community of this project.
1) I doubt that. The people who want to contribute already have ways to do so (and they use them). I would even argue that creating a patch file is considerably easier than going through github's convoluted pull request process.
2) We are very happy with hosting the code ourselves. We didn't move away from one free hoster just to see the next one being crushed by its popularity.
3) We are very happy with using SVN.
That said, there may be a git mirror in the future, this has been planned at some point but not turned into reality yet.
And don't get me started on git
(also, most people confuse "moving to github" with "moving to git" - github is not the only free platform offering git, and github does not only offer git repositories.)