To my western knowledge they are called light novels because they are
- printed in A6 making them physically light.
- they use lower level Kanji (the more common ones) that are easier to read for the young target audience.
The subject matter and quality of prose have nothing to do with the term "light novel" as far as I know.. But let's face it, when a customer picks up a light novel, they have certain expectations. And they're not the Divine Comedy or War and Peace.