What kinds of problems did it have? Why is it so difficult to support MySQL and Postgres at the same time, and why would MySQL be prioritised?
"Because it's easier to slap a system together on MySQL". Which, to be fair, it is but that's not a good thing when you're talking about data.
The model it replaces/improves on is self-host on shared-hosting web hosts. That used to be php and mysql - I guess they kept mysql, even if nodejs doesn't lend itself to the same type of shared hosting as php does (in fact, neither does php, but that's a battle for another day).
Anyway, many hosting services will throw in a database instance on a more-or-less well-managed mysql cluster - at a minimum there might be some semi-regular backups of the database, with some automation for doing restores (and with more than a bit of luck, they'll keep snapshots, so that the sql injection that wiped your database haven't resulted in all-empty backups by the time you realize).
I don't think it makes much sense, but I doubt it is because "postgres is hard, mysql is easy".
It's like Rails - it's easy to slap something up but once you start using it in anger, it requires all the handholding and effort.