Maybe it's because Postgres is easier to write extensions for? I do think MySQL is still used more widely, but since people on HN and other hacker/tech circles write extensions, you hear about Postgres more. Those extensions also increase Postgres's actual usage, like I know people who only know about Postgres because of PostGIS.

Regardless of popularity, idk, Postgres feels nicer to use for me.

n.b. PgDog isn't an extension