It's simply not that widespread of knowledge. Modern Postgres users would never suggest Mongo, but a generation of engineers was taught that Mongo is the NoSQL solution, even though it's essentially legacy tech.

I just ran into a greenfield project where the dev reached for Mongo, and didn't have a good technical reason for it beyond "I'm handing documents". Probably wasn't aware of alternatives. FWIW Postgres would've been a great fit for it, they were modeling research publications.