I think it's more economics then anything else. To the business manager, a truck is a truck. He could care less what brand he buys, and what motor is in it, it's all about cost per mile. Most go with Ford because they are cheaper and offer great fleet pricing. As for the engine, there is not enough difference to warrant the extra 5-6 thousand dollars to go diesel. The large gas engines get almost the same as a diesel when towing, repairs are cheaper, and fuel prices are also about the same. Most fleets are leased, they do not care about long term, 3 to 4 years and they will turn it over, and get another one. What brand they replace it with will most likely be determined by who has the best deal. The residual value at terms end for a diesel is not enough to offset the extra payed, you shell out 5-6000 for the diesel, and get only 1000-1500 back. There are some contractors that do run diesels, most that I know of are in the construction business, mainly excavation. Why do they use them?? I would say it has something to do with the fact that they work mainly with diesel power equipment and feel comfortable with them. The fact they they might run off-road diesel fuel through them might also have something to do with it also??
Put yourself in the position on a bean counter or owner. You maintain a fleet of 100 truck, and turn-over 33 each year. You buy all 2500 series trucks at $21,000 each or $693,000. 00 . The same 2500 series truck with diesel cost you $4,500. 00 more or $148,500 for the bunch. In lease terms the extra $4,500. 00 more per truck will run you an extra $4,125. 00 every month. Also factor in the extra insurance cost, and you will see that diesels don't exactly make the best choice for many business's.
For the normal truck owner, diesels are purely an extra luxury. You don't realy need the diesel, nor will it ever pay for itself, in fact most would contend that they spend much more on their diesel truck then they would have if it were a gaser. Most seem to view their diesel truck as a well earned bonus, something they always wanted. It almost becomes a hobby for some, me included, I figure many add all the newest duedads because they feel like their truck will be around for awhile, and they are probalby right. I sure hope so.