i think part of it was customisation, since you and most other kids would likely have the same books from the booklist so it would be easier to tell each one apart when you're in class

also i think another part is that school books are going in and out of bags a lot, and the exposed thin (in the case of softcover) or thick (in the case of hardcover) cardboard was liable to get damaged/frayed/bent, and so giving the cover a bit more structure cos of the plastic doesn't seem like a bad idea