Books into movies

Jodi Picoult is one of my absolute favorite authors, so I was very excited tonight to be able to watch "The Tenth Circle" on Lifetime. Why do they do it? The story wasn't changed too much, but there was just so much left out of it that I wanted to get my book for reference through the movie. I actually know to read the book after the movie, I learned this early on after watching "Lord of the Flies" after reading the book in jr. high, or high school - I can't remember, but you would think I would know better by now. *remember, always read book after the movie*