It is very simple: The "bean counters" in Marketing!
THEY don't think a FRED will sell, so the company won't make them.
Why is it difficult to find a standard cab pickup? Marketing!
Why is it difficult to find a 4X2 pickup? Marketing!
Why are the majority of new pickups on the lots 4-door cab? Because Marketing says that is what MOST consumers want!
Why are most (in some brands, ALL) pickups automatic transmission, a manual is no longer available? Again, Marketing says very few people want a manual, so we won't build ANY!
Remember the "bad old days" when you could walk into a dealership and order EXACTLY the vehicle you wanted, built with just the options YOU wanted, if you were willing to wait for it? I guess you still can with big trucks, but that's about the only thing.
