There is actually a fair amount of competition from Japan. The Toyota Tundra/Tacoma, Nissan Titan/Frontier, and Honda Ridgeline are all popular and in some ways better trucks than their American counterparts. I think many "truck people" are also people who tend to prefer domestically-made goods; in the same way that people shopping at Home Depot are more likely to buy products with the "Made in the USA" sticker on them.
There's a bit of irony in that the Japanese car companies make all their cars (destined for the US market) in the US, whereas the American car companies tend to prefer Mexico.
