Why does everyone run ancient Postgres versions?

Upgrades are hard. There was no replication in the before times. The original block-level replication didn't work among different major versions. Slony was a painful workaround based on triggers that amplified writes.

Newer PostgreSQL versions are better. Yet still not quite as robust or easy as MySQL.

At a certain scale even MySQL upgrades can be painful. At least when you cannot spare more than a few minutes of downtime.

I've always wondered why Postgres is so insanely popular. I mean it has some nice things like very powerful support for a very comprehensive subset of SQL functionality, but most apps don't need all that.

It really feels like early 1990s vintage Unix software. It's clunky and arcane and it's hard to feel confident doing anything complex with it.