The reason is it's a completely unrelated product (previously known as "stash") that Atlassian pretends is somehow connected for branding reasons.

Yep, the two products are even written in separate languages!

Bitbucket Server is written in Java and bitbucket.org is written in Python.

I was dumbfounded when I found out they were two separate codebases.

I thought it's like JIRA. Or is JIRA self hosted also different from the cloud version?

I don't understand what you mean by "like JIRA". Remember all these Atlassian products have been acquired as what were originally separate companies.

...grumble grumble...Skype for Business...OneDrive for Business...

(hate it when companies do this)

It's because mercurial is legacy, like darcs, bzr, monotone, and the other zoo of "git-alikes" that were big in 2008. Now it's just like MacOS Classic in 2000: there are a few die-hard zealots who claim that the interface is better in some indescribable way, and everyone else has moved on.

Does this look like the commit log of a legacy product?

https://www.mercurial-scm.org/repo/hg/

Mercurial is very important for Facebook, Mozilla, Google, and others. The disparaging "legacy" monicker might be a good way to feel justified to not have to learn about it, but it's not dying software that is no longer getting updates.

Speaking as someone that works with Mercurial on daily basis, and also makes/sells software that supports Mercurial. I can tell you it's far from beeing legacy.

What determines if a technology is considered legacy is not if there exists someone out there using it, but rather whether there are people actively adopting it at present. In this regard, Mercurial has slowed down and Git has clearly won the game.

There are companies adopting mercurial as they VCS of choice, either by moving out of svn, or choosing a new VCS