This project is managed via mercurial which I like more than git
I don't think that centralization which is now happening (most projects are hosted there) is a good thing. Now if Github has problems, or is blocked in some parts of the world many more users have problems. Decentralization is good.
I don't believe that projects get support merely because they are hosted on Github. Project gets attention because somebody is interested in contributing to it, and this happens because somebody is interested in the project or thinks that it is important. Platform is a tiny factor especially if we are comparing mature and feature rich platforms like Github and Bitbucket.
Thanks for your detailed answer, these are all valid reasons.
Just a few comments:
That's a good point, but so does BB. At least it's only a matter of website, issues etc., as the code itself is decentralized anyway – that's what VCS like Git and Mercurial offer.
Another aspect: Neither Github nor Bitbucket are FOSS or open themself. Therefore, for me personally, GitLab looks more and more a reasonable alternative by now.
That's true, once someone wants to contribute (or is interested in any other form) it doesn't matter, where the project lives. However, first someone has to know about it and it's common these days to look at Github first (that's what I meant with more attention).
Both platforms provide the basic facilities like VCS, Issues, Wiki etc., though on Github you get eg. free and easy to setup CI systems, automatic code analysis and more.