I can think of reasons.

I have a personal dislike of mongodb, for many reasons but I’m going to put them aside for a moment.

MongoDB beats PostgreSQL because it’s relatively easy to shard and scale. If your application logic isn’t quite capable of sharding then mongodb handles this and replication for you.

It’s also true that mongodb attempts quorum over consistency. Which is just fundamentally different than how PostgreSQL works, it is possible that mongodb can be faster in a very large mesh topology.

What I am hoping for in future is that something like elasticsearch exists but with PostgreSQL as it’s engine instead of lucene. Such a system would be a complete death knell to mongodb.

Doesn't seem to be a whole lot of people trying it out, though the project is 'relatively' active.