There are plenty of die-hard fans, it's just a niche. Nowhere on the X-Files scale, that was a big cultural phenomenon. Animorphs was an adolescent book series which a lot of kids liked, which was curbstomped by Harry Potter after that reared its head.
Maybe not fair, but hey, who'd want Animorphs to have become a big Twilight-style thing anyway? It still did very well during its time, second only to Goosebumps among the Scholastic stuff before Potter went nuclear.