JSON wasn't stable in Postgres until 2012, whereas the "web scale" memes for Mongo started after its initial release in 2009. Mongo indeed had better marketing and a first mover advantage, and it never let go.
Early versions of mongodb locked the entire database server for every single write. Later versions locked the entire database for every single write. Only since Mongo 3.0 does MMAP only lock a single collection for a write and is WiredTiger available to offer you MVCC. And even in the MongoDB world materials it advertised now you can use more of your hardware.... Granted you could always shard, though that gets very expensive very quickly. Meanwhile postgresql had MVCC all along and its write speed was always faster than mongo on a single server basis. And you always could serialize data that changed into text fields as xml/json/csv/some other format. Mongo was mostly great marketing and a lousy product. Over time it has gotten much better. Mongo 3.2 is a way way way better product than mongo 1.x. But marketing did capitalize on a lot of hype which made no sense. Developers enjoyed just serializing their objects (the ones who didn't think of doing this into a text column in a relational db) and for the ones who did not need 'web scale' they had no idea about the severe concurrency issues introduced by mongo's writes. And for the rest there are plenty of posts about hacking around it or switching from Mongo to something else.
