Anybody who has actually seen the movie will probably confirm that the biggest reason is that the movie sucks. If the movie was actually good, Disney might overlook the racism (see: Dumbo). But SOTS isn't just racist, it's mostly just boring. The best part is the animation, but that's only about 15 minutes in three separate parts over the course of 100 minute movie. And while there's some good moments, those 15 minutes are nowhere near Disney's most memorable.
If Disney hadn't tried to hide it, it would be as forgotten as any of the other dozens of boring live action movies from that period Disney put out like So Dear To My Heart and The Sword and the Rose.