Because a huge chunk of the story is wandering the gorgeous, but empty desert with a bunch of psycho killers. Occasionally that group commits grisly large scale harvesting of passable scalps to sell off to bigger towns with a scalp trade. Sometimes one of them, usually the judge, will commit a little extra horrific crime against humanity as a treat for themselves. The "good guy" isn't exactly someone to root for either. It's a story with muddy, dark morals and an ending that'll bum out a lot of folks. No happy days here.

I loved the story and would watch a well done movie about it. I highly doubt that'll happen. To do it right is to include almost all of the horror of what these people are, which would be a lot of money on effects that will anger a ton of people due to what they portray. It's not that it's unadaptable. It's that it would be a slow burn movie with brief, hyper violent hollowing out of small villages including baby smashing. It's slow, mean, and ends in a way that'll have you stare off into space feeling a little bad about the nature of humanity. Not a very profitable idea for a movie.