The other reason is Bart.
I'll make a case for Bart. He ends up with the strongest, most complex characterization in the entire book, leaving the rest of the cast looking like indecisive, oblivious milquetoasts with their heads up their asses. A lot of people find Bart creepy and off-putting, which is absolutely the case, but there is no denying that he is indeed Cathy's kid.
If There Be Thorns is arguably Andrews' most ambitious novel thus far. She's writing from the perspective of two very young characters. There are two narrators who alternate chapters. And it's the only Andrews' novel with male protagonists.
That being said, all these things are executed pretty badly, though not all that much worse than in any of her other novels. I've complained before that Andrews is terrible at writing realistic children, and males, and indeed human beings. When all these elements are pushed to the forefront at the same time, the weaknesses are all the more obvious.
Yet on a technical level, this is probably one of Andrews' better novels. Since she's dealing with a shorter period of time than the other books--a single summer, rather than years--a lot of the pacing and continuity errors that were so confusing in the first two books vanish. And at times she gives a very clear image of the separation between adult-world and child-world, and the uneasiness of children who realize that their parents are untrustworthy even while they are totally dependent on said parents. These are themes that were very prevalent in Flowers in the Attic, but here, she seems to get further under their skin.
Shall we start this thing? Let's start this thing!