The price of persistent storage and memory was the same for Mongo as it was for Postgres back when the NoSQL movement happened. Mongo wasn't made of magic scaling fairies and still needed resources. As soon as you cranked up the safety of Mongo to a nearly acceptable level it's performance fell through the floor. The only reason people though it performed amazingly was because of Mongo's rather deceptive marketing.

I use the term "nearly acceptable" because for a long time Mongo's data consistency was absolutely crap (https://jepsen.io/analyses)

Personally I think people used Mongo because it was new, shiny and exciting and it promised performance (though it didn't really deliver for most people).