Otherwise you have to repeat a lot when you're filming. And I've seen the film and it's in a border town in Mexico close to the border with the United States, so of course you have a lot of traffic and noise and dogs barking right nearby, and for that you have to repeat a lot and Robert Rodriguez didn't do that. Smart, smart, smart, you better have smarts also.
When you're reading a book, you're always looking for the natural place to stop. With a movie, you can't really have that sense of it coming momentarily to a halt; there's pressure to keep the momentum up.