I would argue that MongoDB is not—and has never been—the
best choice for solving any particular technical problem. But it had some other "advantages" over other, better solutions – in that it was easier to set up, didn't require schema definition, had a passable clustering story etc.
I have worked with at least one company that had been built using MongoDB as a primary data store from day one. This caused untold pain later on, but the trade-off is that it likely allowed the company to exist at all – the founder being more of a domain expert than a technical expert, but being able to use it to scale their idea pretty quickly without having to pay much attention to all that tedious "reliability" and "safety" nonsense :)
That said, it's not something that an experienced developer should be using for anything nowadays, and the solution might be to ensure that competing alternatives (like Postgres) can learn from why MongoDB became popular and seek to solve some of the pain points in their own implementations.