It might be better to wonder what are the advantages of mercurial over git.
Better Windows support ? I think a huge proportion of OSS devs these days use linux / osx.
It might be useful for mercurial to focus on making itself the premier OSS DVCS for "enterprise"; that would create space between it and git / the git userbase, and would allow for it to specialize in a meaningful way that is perhaps less-easily forklifted into git.