As I understand it, the main difference is that MariaDB was forked from MySQL 5.7 and therefore still contains the query cache which is beneficial on some workloads.
Also we have seen some compatibility issues with importing cPanel backups which were generated on MariaDB servers and therefore it's important that we support MariaDB for users who are doing mass migrations from cPanel so they can ensure the best possible compatibility.
The reason it's been delayed is the complexity of supporting MySQL 8 and MariaDB side by side.
