You would use it over PostgreSQL because it's a drop-in replacement for MySQL. PostgreSQL is not.
If you're not using MySQL in the first place, why would you care?
If you're not using MySQL in the first place, why would you care?
However, for MySQL in an existing application, the migration costs might make it not worth it. There can be a number of pernicious gotchas that aren't obvious at first. MariaDB is more or less set-and-forget.