First note that, as others have said, removing a book from a required reading list is entirely different from banning it. Nor can we know why it was removed from that particular school reading list unless the people responsible for removing it have stated their reasons. If they have (and you believe them) the question is trivially answered, and there's no point to discussing it here. If you don't, we have to speculate.
Perhaps we can approach an answer by asking why that particular book (or any book, really) should be on so many other high school reading lists. Could it be because many of the people responsible for those lists liked the socio-political ideas it expresses? It surely, at least from my memories of high school English, can't be because of any inherent literary merit, or appeal to the typical teenaged mind*. I recall it as boring & tedious (though not nearly as bad as that other favorite of English lit, "Catcher in the Rye"), yet it was required reading in my high school English class, less than a decade after its publication. It evidently remains there, while other books of far more lasting popularity (and IMHO much more interest) from the same period are ignored.
So why do some books that aren't) of obvious (at least to me) interest to most high school students placed on reading lists, and why are others of much greater interest (as judged by sales and/or library borrowings) ignored? Shouldn't at least one goal of English be to get kids to enjoy reading?
I suggest that a large part of the answer to both questions lies in the socio-political ideas expressed in the book. People who wanted it on reading lists starting in the 1960s essentially wanted to use it to spread an anti-racist message. People who try to remove it now seem largely to object to the language of the period as perpetuating racist attitudes. (As with the objections to "Huckleberry Finn".)
*Granted that I probably wasn't a typical teen, but my choice of reading matter back then included Heinlein, Tolkien, Stout, Christie, and pretty much all the non-fiction in the library. I read them because I liked them, not because it was a class requirement :-)