Git won in the same sense that chrome won or windows won, there's value in having alternatives and I would like to have the option of mercurial in more settings.
I am not advocating for use of git or mercurial, use whatever suits your needs! But I think having a platform like bitbucket support alternatives to git is a big plus for developer freedom to chose.
I think another interesting VCS is probably Fossil. It's very self-capable and self-contained. I wish it were a little more commonly used, but git or mercurial is where it's at I suppose. I keep meaning to use mercurial more, maybe I should for my BitBucket projects, it's not like it makes much difference since JetBrains IDE's handle the under the hood version control commands.
I am not advocating for use of git or mercurial, use whatever suits your needs! But I think having a platform like bitbucket support alternatives to git is a big plus for developer freedom to chose.