Because Don Bluth became a bit of a persona non grata after he “rebelled” from Disney Animation. Most of his films (American Tale, Secret of Nimh, All Dogs..., Land Before Time, Rockadoodle, Thumbelina, Cats Don’t Dance) struggled to match the marketing and distribution abilities of Disney and, because they also were much darker thematically, didn't grab the public consciousness.
Damn shame. I adore Don Bluth.