Why are Samuel, Kings, and Chronicles divided into two books each?

In earlier times books were written on scrolls -- long strips of parchment or similar material. The strips were wound around two sticks. This works well for small books, but large books become cumbersome. Imagine trying to look up a passage that you know is about two-thirds of the way through a large scroll. You would have a lot of winding and unwinding to do to get to the part you are interested in reading.

Each book of the Bible is represented by a scroll. Some of the really long books were kept on two scrolls for convenience and that separation continued when the books became bound as we bind books today.