If you have ridden in a DP you have the answer - the ride. Plus, if you are out west, climbing & coming down mountain passes, the exhaust brake just walks you down the pass so easily - again it contributes to the ride.
Next we have carrying capacity - - most DPs have a larger CCC - we had over 5000 lbs available when we started, plus big bays that are open clear across the rig.
Then there is the fact that the engine is in the back, so it is quite and you can listen to music, talk, all without that engine between the two of you.
As to maintenance costs - yes it is more, but the interval in between is longer. If we spent all of our time in the midwest or east, then a gasser would do fine, but we cross the Rockies & Cascades/Sierras at least twice each year - usually more than that as we wonder around for the summer.
Barb