Popularity is a strong argument. It's however not the only argument. In the case of git and hg popularity is important because the other arguments are too weak

Popularity is a strong argument. It's however not the only argument. In the case of git and hg popularity is important because the other arguments are too weak to matter in comparision. That's not the case for GNU Octave or Linux. Those offer significant financial, ideological and technical benefits over alternatives. Benefits a large part of their user base actually cares about. Hg doesn't have that luxury. Git is simply good enough and hg is not good enough to offset the cost for choosing the unpopular option.