We use MariaDB because it is a free open-source SQL database. I was looking for a low-cost technical stack, and SQL database made a huge sense as it is transactional, and offers all the confort of relational queries, which, for me, is better for general purpose database than noSQL alternatives.
MariaDB is the really free drop-in replacement for mysql, which makes it a good default choice. Note that at some point, I plan to support also Post/gre SQL, which is the other famous SQL open-source database in the enterprise world.