Because GitHub has graciously offered us the hosting. Also, because GitHub is more than just storage for git repos, it is also a social network for code. Lastly, GitHub's API allows for a pseduo-API to GITenberg for free!

But the project is GITenberg, not GITHUBerg. If we could provide better tools with a separate website that we write and maintain, then we may.

Also, putting GITenberg on GitHub means that it isn't owned by me. If someone wants to fork a book on github, that book is now theirs.