Was there something wrong with PostgreSQL before 2010? We used it and I thought it was pretty great (still do) but maybe I'm missing something.

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.