Defending Jacob

I'd only read the book a few weeks before this started so the plot was still fresh in my head, I knew there would be changes but I was really hoping they wouldn't make it completely unrecognisable (Virgin River did this). I was pleasantly surprised that for the first 7 episodes the changes weren't too bad at all, Andy and Laurie were much younger than in the book, Laurie was a homemaker, formerly a schoolteacher and Pam Duffy and Joanna Klein were both originally male characters. There was more but nothing that would have made me switch off in a huff.

Then we got to episode 8 where they completely changed a major plot line that was essential to our understanding of both Jacob and Laurie. Then they changed the ending. So the 8 I was originally going to rate it went down to a 5.

Why are final episodes so often a let-down?