For a side project, there are advantages to choosing something popular. If you're building a side project to bulk up your resume, choosing something popular can help you land a job (as there are more available).

If you're just trying to learn stuff, there are more people to help you out and more answers on Stack Overflow.

I'd rather use the technically good-enough solution with a large ecosystem than the technically perfect solution without one.

Are people choosing Angular for the same reason?

Widespread usually means a lot of people are familiar with it and since the community is large, problems should arise and be fixed quickly - in the best case.

Not advocating Mongo here, just saying that adoption is not an irrational reason. I have mostly seen this with Java, when people tell me: We chose Java, because that's what they teach at universities and this is what developers know.