There's no reason not to use Mercurial. It's version control software. If you like it, use it. It's that simple.
By definition, version control is for collaboration, so using a less well-known version control system impedes the regular work of your project.
There's also the issue that version control is integrated into lots of other software. Everything now supports Git and Subversion, but support for other VCSes is rarer.
This is kind of unfortunate, because it makes innovation hard. Projects like pijul exist to try to move the state of the art forward, but it's difficult to see how they will gain adoption.