Because ease of learning
Because community and support
Because frontend and backend
To make things simpler, many NoSQL databases use the same key-value representation which means all of your data, from the database, to backend and eventually the frontend, is in one format. It makes things easy to understand. It shouldn't then be surprising that many of the bootcamps choose such a NoSQL database to go along with their full stack development course.
Because it works (..for now)
There are only two kinds of languages: the ones people complain about and the ones nobody uses. - Bjarne Stroustrup
That quote is particularly appealing to me, and not just because I had a mini-crush on Stroustrup ever since I missed his talk back in 2015.