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.

Is slony some sort of language joke? Slon is czech (probably slavic in general) for elephant.

(which may be actually derived from the turkish aslan, for lion, but somehow the animal got mixed up)

Not really, the older form of Turkish aslan is actually arslan.