I conjecture that it's rare that someone just needs a document DB.
The main problem someone is thinking about may be solved by a document DB, but then they also have 1000 other problems they didn't think about that are a horrible fit for a document DB.
And due to the nature of databases, where data has greater value when combined, using a new database system for every application isn't a great option.
SQL is great for business data, and passable-to-good for everything else. So the only way to beat it is by being way better at some specific thing, and finding users where that specific thing is so important that it's OK if the data is on an island.
The main problem someone is thinking about may be solved by a document DB, but then they also have 1000 other problems they didn't think about that are a horrible fit for a document DB.
And due to the nature of databases, where data has greater value when combined, using a new database system for every application isn't a great option.
SQL is great for business data, and passable-to-good for everything else. So the only way to beat it is by being way better at some specific thing, and finding users where that specific thing is so important that it's OK if the data is on an island.