At some point during the past 10 years or so, the world realized that document databases are actually a bad idea, and relational databases can do the same job better with just a few easy-to-implement QoL improvements (such as JSON operators in SQL).
Meanwhile MongoDB's creators thought this was a great time to make Mongo non-free software, presumably with the goal of making a quick buck from cloud operators.
The result is pretty much the landscape we are seeing today, with Postgres reigning supreme, and even SQLite being experimented with in server setups, while Mongo is on life support and MySQL not far behind.
Meanwhile MongoDB's creators thought this was a great time to make Mongo non-free software, presumably with the goal of making a quick buck from cloud operators.
The result is pretty much the landscape we are seeing today, with Postgres reigning supreme, and even SQLite being experimented with in server setups, while Mongo is on life support and MySQL not far behind.