Simple, diesel costs more per gallon, it's not as widely sold compared to gasoline (not every station may have it), and there are less car models availavle with

Simple, diesel costs more per gallon, it's not as widely sold compared to gasoline (not every station may have it), and there are less car models availavle with diesel as an option. Therefore, even if you just look at it statistically there will be less diesel cars on the road in the USA.