Why have books been inappropriatley banned in the past?

Because those in authority believed the contents of the books were detrimental to readers and/or the society in which they lived. Usually this meant that books the establishment felt were too violent, sexually graphic, or promoted behavior or political views that were out of the mainstream. This is exactly why the framers of the US Constitution included freedom of speech (including the written Word) in the Bill of Rights. The opinion of one adult should not prohibit another adult from expressing his or her views on any subject or for others to be able to know that opinion.