I think a big part of it is a general fear of database migrations. Once you embrace schema evolution as a normal part of development, it becomes natural to have your schemas reflect your true data constraints and relationships. Rather than just unlabeled sacks of text fields.
I remember working on my first full-stack project, I was so intimidated by migrations. Nowadays I couldn't imagine starting even a small side project without writing migrations from the first commit.
Unfortunately, I think a lot of new developers resort to MongoDB because they think it's better for early stage evolving schemas. It's certainly more open to changing your schema on the fly, but perhaps not for the better.
