The reason has to do with the length of the driveshaft and its ability to transfer power to rear wheels. Many of the old stigmas of the smelly, hard-starting, noisy diesel engine still apply, but their effects have been greatly reduced. Today's diesel engines are quieter, harder working and more efficient than every before, as best exemplified from the latest offerings from Cummins and Caterpillar. With the ear-mounted engines of today's diesel pushers, noise is simply not a negating factor. This is a different story for your tow vehicle, however, where you're just a matter of inches from an engine that isn't afraid to howl.
Cost is another factor.
Diesels are simply more expensive to buy, whether it's for your RV or tow vehicle. Unfortunately, winter creates another set of fuel problems for diesel owners. Icing and gelling in the fuel tank, associated with the changing of temperatures, often necessitates special care. A number of fuel additives are available to remedy these problems if they should occur. Travelers also have to be sure their diesel fuel is fresh since it doesn't share the popularity of unleaded fuels, and thus, runs the risk of being old.