I think they were popular because they’re in the format of an “epic series”, but digestible by people who normally couldn’t make it through the first volume of any serious epic series.
Remember what a big fad Lord of the Rings was back in the 60’s, early 70’s?
Imagine Lord of the Rings for the tween set, replete with characters they can objectify and identify with, villains they can understand and reflective of their real or imagined experiences ... that sort of thing.
Throw in the saucy dialogue, clever(ish) wordplay, and you have mind crack for the younger set.