When it comes databases I most commonly hear customers split two needs/wants. 1) data integrity (ACID). 2) Speed.
When it comes to data integrity the obvious choice is to go with a SQL database like Postgres. When it comes to speed the obvious choice is go with a memory db like Redis. Mongo sits awkwardly in the middle where it isn't ACID compliant, and it isn't a memory db so can't outperform Redis.
Another factor is that a lot of things have migrated to cloud managed db's, so if you need a document store, and are going to use a cloud managed db, then something like DynomoDB makes a lot of sense.
EDIT: Apparently Mongo added support of multi-document ACID transactions back in 2018 https://www.mongodb.com/blog/post/mongodb-multi-document-aci... . Who knew? Not sure why but when I think of a database that needs to be ACID, mongo isn't even on the list for me. Seems like just a misconception/myth on my part though.
That should be: Why do so many people on HN dislike MongoDB?
MongoDB is the #5 database on DB Engines and has been in that position for a number of years. [0] In the general developer population it is quite widely used.