Essentially, certain books are banned or challenged because they contain controversial or difficult ideas or information. At the base, these challenges are to protect others, mostly children, from these topics.
Why is this an issue? Challenging and banning books is a form of censorship. Censoring materials, while the intentions are good, keeps these materials away from others that might need or want to read them.
"The American Library Association promotes the freedom to choose or the freedom to express one's opinions even if that opinion might be considered unorthodox or unpopular, and stresses the importance of ensuring the availability of those viewpoints to all who wish to read them."
Source: ALA Banned Books FAQ
The most common reason for books to be challenged is because they contain diverse content.
What does "diverse content" mean?
Books by or about:
Source: ALA Defining Diversity