In the WordPress world, it is the only database.[0] If you do WP, you do MySQL. You may sniff at WP (I certainly do), but it is used for 43% of all websites, which is not a trivial number of deployments, and not a small market.
I suspect it has a large market share in the "started as a trivial PHP program, grew to large application, too committed to MySQL to migrate to something else" market, too. There's that one website with the blue F logo that went this path, but there's probably thousands more.
That's not the same thing as anyone actually preferring it, but that's two possible reasons why there's an awful lot of MySQL deployments out there.
I suspect it has a large market share in the "started as a trivial PHP program, grew to large application, too committed to MySQL to migrate to something else" market, too. There's that one website with the blue F logo that went this path, but there's probably thousands more.
That's not the same thing as anyone actually preferring it, but that's two possible reasons why there's an awful lot of MySQL deployments out there.
[0] https://codex.wordpress.org/Using_Alternative_Databases