I think you either want it to have more functionality and you choose Postgres, you want a particular performance profile and you pick MariaDB, or you want the enterprise support and you pick Oracle's database. Why you would pick Oracle-supported MySQL today I'm not sure.
PostgreSQL still after all these years has a terrible story when it comes to HA and horizontal scalability. Nothing is built in or supported. And one of the only companies to provide something half decent i.e. Citus is now owned by Microsoft and so that project is now at risk.
At least MySQL has something built-in.
With MySQL group replication, it is pretty easy to set up a HA cluster that allows you to sleep soundly. It can also handle thousands of connections without the need of a connection pooler.
Among the largest users, MySQL (or Percona Server) is chosen far more frequently than MariaDB.
GP mentioned Group Replication, which isn't even available in MariaDB.