Back in the day (i.e. long before Oracle purchased MySQL), if you wanted an open source database, you had two choices: Postgres, which was reliable, and MySQL, which was fast. It turned out there were a lot more people who cared about being fast than there were who cared about reliability.

Eventually Postgres got fast enough and MySQL got reliable enough, which has made the distinction between them somewhat fuzzier today. But by then MySQL had become the standard database offered by umpty zillion web hosts, and ubiquity is a hard position to dislodge someone from.