They were a fairly early entry into modern document databases. They offer sharding/mirroring, and have a really simple, relatively nice interface. In the end, they're a good (enough) fit for a lot of use cases. While RethinkDB imho is absolutely better, market share entropy counts for a lot. ElasticSearch is a similar product in the end, with lower consistency guarantees, that is another good fit for a lot of use cases. Cassandra (C*) is also a good fit for similar use cases, though more difficult to work with, it's also more tune-able to specific needs.

It really depends on what you need... Every non-sql database tends to sacrifice something for some performance gain... RethinkDB is as close as I've seen to one without sacrifice.