There are a lot of reasons IMO. MySQL has a proven track record for stability and performance powering huge sites. I would say MySQL has a nicer replication story as well.
In general MySQL is a lot more widely used with a greater pool of knowledge out there.
MariaDB is a drop-in replacement for MySQL, is made by the creator of MySQL (after he left Oracle post-Sun acquisition), has more features and is now generally considered the upstream of MySQL. I also bet Monty is more receptive to working directly with your teams to scale the product or make changes as necessary.
In general MySQL is a lot more widely used with a greater pool of knowledge out there.