When I started with PHP3 development in 1998, PostgreSQL was still called Postgres95 and it had some severe limitations which were directly adverse to the features needed for web development:
1) performance was very slow
2) There was an effective row size limit of 8K over the whole row which worked against people who wrote forum software (big posts) or were (ugh) storing binaries in the database.
3) Even today, PostgreSQL requires a bit more maintenance and configuration until it works right, whereas MySQL ran out of the box. Back then, VACUUM wasn't at all optional and still required an exclusive lock over the tables it was working on.
4) There was a misconception that each Postgres user needed to be a Unix system user (this might or might not be true. Probably isn't, but I and others were certainly thinking that), so it was working against the usual cheap virtual hosting.
5) The MySQL extension for PHP was much more advanced than the postgres one, leading to people preferring MySQL which in the end lead to the virtual hosts only installing MySQL at which point Postgres' fate was sealed.
6) MySQL was coined as a spiritual successor to mSQL (just look at the names), which was very popular at the time as it was the first SQL-database for "normal people". You either did flat-files or mSQL.
7) MySQL had Windows support from the beginning. Postgres95 was still using cygwin (if you could get it to work). Back then, many people were using Windows as their development environment and being able to run the whole stack on your development machine does have some advantages.
By now, aside of the maintenance thing, Postgres has lost all these drawbacks, but now it's too late as people are using what they know is working for them, so they are just going with MySQL (or SQLite if they need more simplicity, which is interesting in itself as MySQL for ages provided an embedded linkable and serverless variant which also never took on).