History is not an art because you are not using creativity like in art or music, you are learning about a past. It is like asking why Algebra isn't a English topic.
Art history is exactly what it sounds like: the history of art. Although, since the meaning "art" is a gray area, it would be safe to say that art history encompasses the history of visual culture....
art is what people or you create.like painting,dancing and other arts. but history of art means when was this art got created,what changes has been made and what was the cause...as far as i know...