Can't answer for Quora, but I know why I'm not: activation energy. My team and I know MySQL pretty well, and we can stand by our decision to use it. PostgreSQL we can't.
Postgres is easy to convert to from MySQL, if you keep that conversion cheatsheet handy. However, mastery of its performance quirks (all servers have quirks) can take years.
It took me 48 hours to convert to it, 10 months later and I don't feel like I know much about it.
Isn't running a system for over 10 months without being forced to learn all the innate details a plus? Gives you nice 'look at the uptime of this baby' bragging rights without too much effort ;-)
It's not the most sound technical reasoning.