Why has Harper Lee separated To Kill a Mockingbird into two parts?Plese explain in detail.

There are two main plots that dominate the novel
To Kill a Mockingbird: The children's fascination with the
mysterious Boo Radley, which dominates Part One (Chapters 1-11); and the trial of Tom
Robinson, which covers most of the first 15 chapters of Part Two. It seems a logical
structural form, creating a break between the two primary story lines. Additionally,
Part Two (Chapter 12) opens with Scout's description of Jem's growing pains; he is
approaching puberty, and the age difference between the still childish Scout and the
nearly teenage Jem becomes more obvious. The final portion of the novel (Chapters 28-31)
ties together the two plots: Boo Radley finally makes his appearance when he tangles
with Bob Ewell, Tom Robinson's accuser.