A large part of that is that 10gen are much better at marketing than they are at tech. For example, they like to pitch against the Oracle database, and for certain use cases, MongoDB is better than Oracle RDBMS. But if you were to take those cases to Oracle, they would say well don't use the database for that, use our other product Coherence (which was Tangosol before they bought them). And Coherence spanks MongoDB in every possible way. You could write a bestselling novel about it, 50 Shards Of Grey.
Another example here http://news.ycombinator.com/item?id=4533760
And if you hate Oracle, there are a bunch of free things that do what MongoDB does a hell of a lot better. Why waste time with its cheesy MapReduce when you could have ICE http://www.zeroc.com/overview.html for example?