well, it is not that difficult to convert from one revision control system to another, at least among git, mercurial, bazaar and other equally featureful systems, which includes fossil too i would expect. maybe some of them are more featureful and track stuff that git doesn't so conversion would loose some data, but i don't see that as a problem. (a lesser system that doesn't track everything git has might be an issue, but actually not even that, since all we need is interoperability not full conversion)

mercurial has a plugin to support git, and the reverse exists too. for bazaar as well. interestingly fossil can export to git, and a git repo can be converted, but it can't pull updates from git into fossil. i wonder why.

anyways, the point is, because interoperability exists and is possible there is no technical reason not to use alternative tools despite everyone else using git.