I can say from a web dev's perspective that the ubiquitousness of MySQL is only part of the allure (an important part though). MySQL makes simple stuff very easy to implement, it's a great solution if you just want your database to store your stuff and get out of the way. Sure, like any other DB it requires some expertise if you want performance, but it makes this incredibly simple. It just works and you don't spend a lot of time arguing with the software.

It also has a single feature that I find incredibly cool: it supports automatically generated row IDs. It's been a while since I last tried Oracle and Postgres, but at the time you had to create a second table just for a sequence number and it was extremely awkward to handle.

Could the auto-increment feature in MySQL be better? Sure, for example it would be nice to have an option where the row is identified by a hash value instead of a continuous sequence. But even in its limited form today it beats having to wrestle with the glued-together equivalent as implemented in "real databases".