from second-source information: in the late 90s, just about every database product in the world, from tiny to massive, was tested as a backend to various web sites. Scaling was still being invented. MySQL was simply the fastest (only by microseconds) to return a set of strings to a php page, and php was a fast development environment. Neither php nor MySQL had the rigor that many in the database world demanded, and that was ok. In fact that helped to make the MySQL/php combination fast.

Once the herds picked MySQL (and php) the economies of scale started, and advantages grew. Meanwhile, the proper and safe competitors were generally disgusted (drama word but not entirely an exaggeration) but who cares about them, maybe even a weird positive to the new breed of web devs who were inventing the future.