I think a large part of that simplicity can be attributed to the lack of Windows port of PostgreSQL at the time that PHP was gaining traction. MySQL was the only reasonable option then. And then the community mindset was solidified.
I think it was the need for manual vacuuming. If you were running a small web host at the time picking MySQL over Postgres meant a huge reduction in potential support tickets related to non-vacuumed databases causing customers to run out of space.
That certainly didn't help, but if your DB was large enough where vacuuming mattered, you probably found a cron solution. The other place I saw MySQL win was in single access benchmarks that were scattered across the Web. Postgres would win in concurrent scenarios pretty handily, but people were glued to those serial benchmarks.
