A rear engine layout is more expensive than front engine, so rear gas coach is inherently more expensive than front gas. Buyers with a larger budget want more than just rear engine - they are into the entire package of a diesel pusher - air suspension & brakes, larger carrying capacity, etc. The rest seem to be driven more by price than capability. Workhorse & Freightliner Custom Chassis have tried both front diesel and rear gas, but engine differences alone were not sufficient to motivate RV buyers. They cost more without enough perceived benefit.
Tiffin tried for a couple years to make a success using the Freightliner FRED front diesel, but could not get enough buyers to pony up the extra price. Most buyers went all the way to a pusher instead. Winnebago tried with the rear gas, but no luck there either. However, it wasn't a real good trial because Workhorse never got all the bugs out of their rear gas chassis so most were delivered with a Cummins diesel anyway.
Diesel power comes into its own as the rigs get heavier, say north of 25,000 lbs (dry) or 30,000 lbs GVWR.