[wisty]

MySQL was popular for the same reason No-SQL is - it lacked features, but it was fast as hell with no tuning. Postgres did more, so it had to be slower, and it was tuned to work on minimal resources (and not steal all your RAM). Like MongoDB today, MySQL was tuned to work fast, at the costs of both reliability, and being a poor citizen on a shared host.

There are other factors - you could limit its use of disk space (so hosts liked it), but mostly it was a good key-value store with some database functionality.

People who wanted a "real" database used Postgres. People who wanted to write a web app used MySQL.