Some of those writers you mentioned aren't American, so that's why they're not on that particular list.
I've never read a Louis L'Amour novel. I singled him out because his name on the list grabbed my attention, as I probably hadn't come across it in decades. Was he really a good writer? I have a hard time imagining someone who cranked 'em out that fast being high calibre. According to Wikipedia he published "89 novels, 14 short-story collections, and two full-length works of nonfiction." Could be I just envy industriousness!
Plus, the subject matter. I was forced to read a couple western (or maybe more accurately "prairie") novels in school. Maybe that soured me on the tumbleweeds and cattle-wrangling genre.