Probably not that - but Ghost was born in many ways as "a better wordpress" (or a not batshit crazy mess of old code and not properly namespaced plugins and themes).

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".