Why was the book to kill a mocking bird split into two parts

Part 1 of To Kill A Mockingbird focuses mainly on the children's obsession with Boo Radley. In the novel's early chapters, readers come to recognize Scout, Jem, and Dill as curious children who entertain themselves by trying to get a glimpse at their mysterious neighbor.

In Part 2 of the novel, Harper Lee shifts her focus to the trial of Tom Robinson, which is a clearer example of the social (in this case racial) injustice that was all too common in the 1930s in the South.