If nothing else, you have to install it. There will also be subtle differences between concepts, e.g. git and svn both have versions and branches, but the concepts behave differently. I don't know about Mercurial, but I'm sure they have their own quirks as well.

Also, tooling: I have a VSCode plugin that visualizes the entire graph structure of a git repo really nicely. Right now, I can use that on 99% of all repos to get an overview of the branches, last commits, activity, etc.

If version systems were fragmented, I'd have to look for equivalent tools for every versioning system separately - if they exist at all. More likely, I'd be restricted just to the on-board tools of every system.