So what? There will always be advantages and disadvantages combined with personal preferences of one source control system over another. What preferences you personally have bear absolutely no relevance, as the ducks says it is up to Kovid who works full time on this project to decide where he wants to host it. Now if you can convince him to *move* the calibre source to GitHub that is one thing, but to setup your own mirror and then complain about Kovid quite rightly choosing not to have anything to do with it is utterly ridiculous.
I really don't understand your attitude with this. It would be utter insanity for Kovid to add to his workload to attempt to mirror onto multiple source control systems, it isn't even worth a minute of consideration. From his posts above he doesn't appear to be motivated to move, and frankly if that is the case I don't blame him - he and the many other contributors to calibre have years of experience with his existing setup and numerous scripts etc, so to stall development and put everyone through the upheaval of learning git is not a decision to be taken lightly.