It's because, fundamentally, it's better for most stuff. It has transactional DDL, check constraints, window functions, CDEs, joins other than nested loop, and

It's because, fundamentally, it's better for most stuff. It has transactional DDL, check constraints, window functions, CDEs, joins other than nested loop, and a better focus on safe-by-default behaviour. It also has a query optimiser that is generally more competent on difficult queries. The per-connection cost isn't much of an issue with a connection pool, and replication has improved a lot lately (and is clearly only going to get better). Aside from ease of getting up and running, IMO there's very little reason to pick MySQL over Postgres nowadays.