MongoDB was a subpar product that rode the "nosql" document-store hype, but people following hype don't know about technical quality and this lead to it being used in a lot of places where it shouldn't, with marketing plastering over all the downsides. They've slowly made it better and now it's a rather smooth experience but overturning a poor reputation is extremely hard.

Meanwhile other database systems have further developed and the need for a pure document-store with weak aggregation and not much else just isn't very enticing. It's still good for data where schema is on-read or in your application, and needs completely flexibility like document and media management, low-volume logging, user profiles and sessions, etc.