Mid door DP are becoming more common. I think Thor started the tread with the Palazzo and the drop down bunk over the front seats. Fleetwood (Excursion) and Winnebago (Forza) has since followed with the trend. Usually you'll find the configuration on the smaller (36' or so) rigs. The drop down bunk forces the door change. The bunk makes getting out of the coach a bit difficult, even more so in an emergency.
A gas chassis just doesn't have the frame to bolt the house too in front of the front axle to allow growth in that direction without frame extensions. Plus you would end up with a lot of dead space for an air tunnel to the engine cooling system along with the additional axle weight and a bunch of other engineering problems.
Personally I prefer the front door location to give the unbroken wall for either a better kitchen lay out of a full length couch and such on the passenger side.