It was hard to install for shared hosting, so most aspiring developers became familiar with MySQL first, with resulting network effects.

The client was a bit harder to use (\x commands are harder to memorize than mysql's "SHOW TABLE" and felt sluggish in a local environment (something that's no longer the case AIUI)).

Replication was a third party addon at the time, and had a reputation for being hairy, though I don't have direct experience.

I wasn't using it, but it had a reputation for being slow and for the replication being considerably more broken than MySQL's -- apparently both of those have come a long way since.