Almost every article I've read 11 comparing Git and Mercurial it seems like Mercurial has a better command line UX with each command being limited to one idea only (unlike say git checkout).
But at some point Git suddenly became looking super popular and number of Git submitters on Debian popcon graph (see graph image below) literally exploded.

Source: Debian
What happened in 2010-01 that things suddenly changed. Looks like GitHub was founded earlier than that - 2008.