I wonder how much of that was due to cursed distro repositories.

I will probably never forget the rage I felt at discovering the reason my scripts broke between Debian versions was because "apt-get install mysql" started silently installing MariaDB instead, which was close but subtly different enough. (I am honestly surprised considering an Oracle trademark is involved, no lawsuits were filed over this.)

MariaDB has seen decent adoption, but it's still an order of magnitude less than MySQL.

I think that postgresql's popularity has also benefited quite a bit from Oracle's acquisition of MySQL