The original purchaser was Sun and they presumably bought it, in part, to compete with Oracle in the lower-end market, to ensure MySQL runs well on Sun hardware

The original purchaser was Sun and they presumably bought it, in part, to compete with Oracle in the lower-end market, to ensure MySQL runs well on Sun hardware (which, in turn, sells more hardware), to sell MySQL related support and professional services and so on. Not long after this, Oracle acquired Sun and along with it, MySQL. One of the MySQL founders didn't like this and started a fork of MySQL, based on the GPL'ed source. There's not a great deal Oracle could have done about this. But they weren't just buying MySQL, they were buying all of Sun. Additionally, the fork wasn't exactly a commercial threat for years - it's only now that we're starting to see movement towards adoption of MariaDB by high-profile, large users.