Having To Kill a Mockingbird in 1st person pov restricts the novel. Scout is a young child, so she doesn't understand the world around her. She also completely misses things that an older person (or a narrator) wouldn't, which may lead in important clues being left out.
Explanation: *gasp*! I had to do an an entire six weeks on this book in English! (I luckily saved my essay questions...)