alt Hacker NewsMySQL had some problematic design decisions initially. They might be fixed now, but the impression remained. And later there was the added complication that they were bought by Oracle, so you didn't really know how this would turn out in the end.
PostgreSQL also had more features back then, e.g. the JSON support is very nice if you need to do anything that doesn't neatly fit into the relational model.
Not only where the problematic design decisions, but large numbers of people said for years "nobody will never need/want that anyway so stop talking about it". That is they didn't even attempt to talk about trade offs, you were just wrong if you suggested anything else.
Then people who knew something got involved (or likely were involved all along - but I never followed MySQL so I'm not sure) and fixed those because they matter and suddenly the crowd shut up.