Git is widest adopted, period (EDIT: because of how it blossomed out of linux dev, essentially). I was able to stick to bzr or fossil for a lot of things until github and gitlab basically became where open source work (and even corporate transitioned to some extent!) needed to happen, and the friction of using anything else is just too great at this point in time.

Which is a shame, really, as --fast-import/export is actually not a bad way to transition from one scm to another these days, and I'd personally welcome a little more active competition in the space to evolve beyond the current status quo.