We used hg in a uni project once, since large parts of our team had never used a VCS, and on of our advisors urged us to consider hg. (This was around 2010, so I guess hg improved considerably since then.)

Even considering the additional overhead, we switched to git after a few weeks since the repository was constantly broken, and the two of us that knew git and SVN were desperately searching for the consistency and tools we had in git. At least we could support the team from there on and make it work for them. Not the ideal solution, but the best available to us.

So I totally understand why git is more popular than hg. Until hg could be considered mature enough for larger teams, git already had a huge following, IMO.