For the curriculum, the principle has usually been to pick a technology and teach it instead of trying to cover all our bases. Teaching MongoDB instead of Postrgres is similar to teaching React instead of Angular.
However, freeCodeCamp does have a wider variety of courses available. If you look at their YouTube channel you’ll see a few tutorials on using Postgres in your stack. The articles on /news also cover a lot of material that isn’t part of the curriculum.
The MERN stack is pretty common. I guess it was decided on both because it’s a commonly used stack and because the DB is pretty easy to set up using a cloud based solution.