Because it has queerly gotten into the inner ear of high school freshman English teachers. And once lodged there has stayed put. I have yet, however, to read one of its victims to say, years later, "By gum it really was a good book" or "I think I'll go read it again and see what I might have missed." By universal acclaim of the intended audience it is a stinker. It's just that they're an involuntary audience and their teachers have reasons of their own. No one except HS freshman English teachers give it the time of day.