The mentioned why they dropped it here: https://dev.ghost.org/dropping-support-for-postgresql/
> Without active community support, Postgres has always been, and always will be a second-class citizen. For that reason, we are dropping official Postgres support from Ghost core.
"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.
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.
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".