I think I first started using it at v6 or 7. I liked it could I could run it from a .bat file and spin it up for tests without installing it.
It always worked well, but there are many newer developments around operations and scaling that finally made it much easier to run. Even now there are some limitations with logical replication and horizontal scaling but the main system can easily be installed and running in a few commands to serve most users just fine.
It should be noted that the rise of Docker containers and K8S has also helped greatly in letting users run all kinds of software with minimal work.
Some of the MySQL backup/restore stuff is a lot more straightforward (innobackup vs WAL shipping).
MySQL replication with binlog_pos was a total crapshoot. The reason innobackupex exists (and anything from Percona, to be honest) is because MySQL's default options were so unworkable. The Postgres equivalent is probably something like barman.
MySQL replication was workable before GTID. GTID simply made things a lot easier. GTID was hard to work into existing DB's at places like Facebook, but it we managed to get things going without a DBA in several multi-TB databases (sans downtime).
GTID is over 4 years old at this point. That's not relatively new in the tech world. Especially with the pace of things like Kubernetes and containers.
There are 3rd party additions to Postgres, too. pg_rewind was written by eBay (?) to address the obvious shortcomings of repointing primaries and replicas.
To be fair, PostgreSQL was noticed way before, Estonia's e-nation website was built on it (even the logic!) and I can't honestly remember when that thing has been down. The UI would need a bit of refreshing but I have the feeling if they're going to update it they're going to replace it all with something shittier.
Both for beginners where it was packaged with PHP and available with basically every webhost, and for advanced users that took advantage of the simple and reliable replication. There were a lot of problems with it, but the novice users didnt run into them and the advanced users knew how to deal with it. These decades of mainstream use carry a lot of momentum and many large companies continue to use it because they've always used it (like Youtube for example).
PostgreSQL only recently became much more usable after v9.5 and is now being recognized and growing in popularity.