Does SF have any rationale for choosing Hg instead of GIT?

I would, like you, prefer GIT...

Mercurial is (supposed to be) easier to learn and use than Git. Git is more powerful but also more fragile and tempramental (if a piece of software can be) - it requires greater care and maintenance.

I think the Linux kernel community went with Git because Linus wrote it - it fits his style (e.g. no debuggers perferable etc) of making it complex to get into, so there's a sort of ability and dedication test involved in just getting started in the kernel community.

I'm all for the move to distributed version control but it would have been nice if everyone used the same. Since Qt had already decided on Git, it would have been better for the SF to do the same IMO. So I agree with you both, although not necessarily because I think Git is a better solution.