I feel like part of that was timing. IIRC, when git was already stable and usable as a daily-driver, Fossil was still sometimes requiring that you completely recreate your repo when updating to a new version.

Git certainly had (and perhaps still has) worse user experience, but it worked and felt production-ready, with, of course, one of the largest open source projects in the world using it, and that made all the difference, perception-wise.