As for why it's included over MySQL, it's undoubtedly the fact that MySQL has been owned by Oracle since 2010. Oracle has not been the most friendly to OSS communities in the past.
This is supposed to be about "racism, sexism" and stuff like that, but I don't necessarily see any contradiction. Because it's not unheard of that in troubled companies, questionable people rise to or stay in power that would not stand a chance in a more healthy environment where better candidates don't run away. Selection for the kind of person who'd rather be captain on a sinking wreck than some lower officer on a well run boat and I'd be surprised if there wasn't considerable correlation between those and the scandal that's supposedly the root cause here.
It never stopped, but it did seem to stagnate a fair bit for a while meaning that MariaDB trumped it on some features & fixes. For instance CTE support arrived in stable releases about a year earlier. That of course may not be the case any more.
We have low visibility into MySQL development process (is the bug tracker still closed? I am not sure), so when they shipped 8.0 in 2018 it was quite a surprise. They fixed a number of outstanding issues, including many extremely bad defaults (like using proper Unicode by default). Some things added in 8.0 (like atomic DDL) are still not available in MariaDB. Oracle being Oracle, there haven't been any major releases since.
It makes sense if the distribution no longer includes mysql, and the mysql the distribution included previously was from before the fork: users who already had mysql installed would be automatically migrated to a compatible replacement. It's the same thing which happened with openoffice and libreoffice.
What actually happened is more like they changed their name, and sold the old name to Oracle. Oracle then forked and carried on developing a new product under the old name, while the original team carried on developing the old product under the new name (mariadb).
That is why distros did what they did.
MariaDB is the fork, technically speaking. The distro switch took some time, probably in part because they wanted to wait and see if MariaDB would be suitable / sustainable.
Fedora and openSUSE switched to MariaDB in 2013, in part due to Oracle's license changes and changes to info published in CVEs and because they were considered easier to work with. (See: https://lists.fedoraproject.org/pipermail/devel-announce/201...)