Why was sounder a banned book?

The title is misleading, it is a correct usage for a telegraph receiver that sounds out the Morse Code messages. In the book it is the name of a Dog. the Dog remains loyal to the family but the Father is a bad man, maybe an escaped convict. it deals with death, suffering, and other unpopular subjects in a rather ugly way. I believe the lead characters are Black, and as stated before not really good guys, is that enough?