[sjs]

For many people, including hosting admins, MySQL was the path of least resistance. MySQL required less maintenance than Postgres back in the day. It's pretty much exactly the same situation as with PHP. The tool created by mere mortals is usable by mere mortals, so they use it. The tool created by scholars is technically superior at core functions but is slightly more difficult or cumbersome to use so is used by fewer mortals.

One thing people seem to forget is that Postgres has not been around longer than MySQL, both were released to the public in 1995. Postgres skipped a few version numbers so maybe that makes it seem older.

[stonemetal]

Counting public releases of non university projects sure, but at least according to the wiki page it has been under active development since 1986, with first release in 1989. Where Mysql started development in 1994.