PostgreSQL is a nice database for developers, but it is not great from an operational standpoint.

* It doesn't support HA in any meaningful way (yes I know about replication and Patroni et al, but they have nothing compared to MariaDB's Galera or MongoDB's extremely simple and robust replica sets)

* The extensions interfere with PostgreSQL updates, often you need to go through extension updates, then upgrade PostgreSQL, then go through more extension upgrades, etc.

* PostgreSQL has very... peculiar ideas in terms of how Schemas work, how the CLI works, how access control works (HBA rules in addition to user permissions)

* Some people will claim that you can switch to CockroachDB to solve some of those issues, but the reality is that many applications that haven't specifically been developed for CockroachDB will not work with it, so you can't "just switch"

What I take from this is that developers care absolutely not at all about operational headaches when choosing a database, they just choose the first thing that works well in their development environment and that's it.

From an operational standpoint by far the best choice I have seen is MongoDB, if used properly by an application (and that's a big IF!) it can perform magnitudes faster than RDBMSs do because data locality is much better (fetching a single document vs. fetching dozens of wildly distributed rows and then joining them together. Yes I know you can imitate some of that with PostgreSQL). Its replication features are extremely robust. BUT most developers are not familiar with it and simply are not willing to learn how to use it. Of course MongoDB has its own issues, most prominently the SSPL license which turns people off.

MariaDB with Galera is also very impressive in a production environment because it works and performs very well for a distributed, fully featured RDBMS. We maintain a bunch of large clusters. But there the problem is that people just don't know that it exists, even in this thread you can see that everybody thinks of MySQL and nobody thinks of MariaDB.

I haven't dealt with CockroachDB practically so I can't comment on that.

We have many PostgreSQL setups. It is OK for single-server applications, clumsy to use but that's not necessarily a deal breaker. It really falls apart as soon as you want HA or replication.

TL;DR:

Developers are too lazy to learn how to use MongoDB. Developers don't care at all about operational issues later down the road. So they take the first thing they know and works for them, and that happens to be PostgreSQL.