Because MongoDB is chasing a moving target. Hype, built in sharding and built in automatic failover are what it has now. The PostgreSQL project was strong when Mongo was conceived and has been accelerating. PostgreSQL has already gobbled up and improved on other MongoDB features like BSON and is where you go when you grow out of MongoDBs more primitive document model. Easy sharding and failover will fall too, years before MongoDB catches up with other must have features like transactions (you will need them eventually) or a half decent query language so you can actually maintain your code rather than having to plan your own queries and encode them as a sequence of JSON documents.