I googled and from what I could find it's due to PC concerns, I'm guessing the 'n' word appearing in it has something to do with the move.
I don't agree with it being removed from schools at all though, I think presented to the right age group it could serve as a snapshot of how things WERE in America at that time without being seen as condoning the things IN the book. Our history is our history, we can't, and shouldn't, censor out the unpretty and unpolitically correct things. We should remember them and learn from them.
Pagan lovin' WOW playing mum to 5 boys in the wonderful land of Oz ... FOR THE HORDE! hehehe