What are some of the reasons this book is challenged so much and was banned in the past?
There are a great number of books that have been challenged and/or banned. You did not state the name of the book. However, you can find information about banned books on several American Library Association (ALA) websites. One ALA site distinguishes between challenged and banned books. It also discusses the reasons for banning or challenging books. A second ALA site is devoted to Banned Books Week, an annual event celebrating the freedom to read.
A third website, banned books week (see link below) has some interesting features including a map of the United States showing locations where books were challenged or banned.
I hope you enjoy learning more about banned and/or challenged books, that you read a few of them, and most of all that you celebrate your freedom to read.