MariaDB is moving pretty fast in regards to adding more modern features seen in other DB's. Such as JSON data type, NoSQL data engine interfaces, etc. As well as spending time working on the current data engines, such as Aria.

While I believe MySQL is as well, it's hard to see progress/what exactly they are working on.

Pretty much the main reason is Monty is working on MariaDB, not MySQL. And he's pretty amazing.