Git has more functionality than Mercurial. For example, there is no Mercurial equivalent to Git's rebase. Whether that functionality is good or dangerous is in holy-war territory.
But to answer your question, I think it's as simple as this: Mercurial has a more familiar syntax (particularly for SVN users) and is fairly well documented. Once you get used to the Git syntax, you'll find it as easy to use as anything else.