As far as I am aware there is no good reason to not use MariaDB instead of MySQL. The creator of MariaDB, Monty Widenius, was on a episode of the changelog podcast if you want to hear some background as to why.
Monty is also the CTO of the MariaDB Corporation, which is a VC-funded entity. I would fully expect him to sing MariaDB's praises in a podcast.
Large companies have come to a different conclusion, at least so far. Among companies with very large database fleets, there's much greater usage of MySQL and Percona Server than MariaDB.
Granted, Linux distributions have leaned more towards shipping MariaDB by default instead of MySQL. This makes sense given that Oracle is, well, Oracle.
We switched to MariaDB on freebsd 2 or 3 years ago. We ran into a couple performance-destroying bugs and while the devs acknowledged the issues, they made it clear they wouldn't be fixed any time soon. MySQL has been solid for us though.
I've been to a Monty talk about this subject, and let me say it was interesting...
Monty starts by saying that he does not acknowledge MySQL 5.6's existence since some new features are an abomination (usually citing GTIDs).
He then compares MySQL 5.5 to MariaDB (of which MariaDB is based on MySQL 5.5; so it contains the superset of features).
MySQL 5.5 was released in 2010. Support for it will end at the end of this year.
Development has diverged a lot since 2010. For a more balanced comparison, I recommend this one: https://www.percona.com/blog/2017/11/02/mysql-vs-mariadb-rea...