I suspect is it also due to the location of the doors in travel trailers. Most have a door roughly 1/3 of the way back from the front and many have an additional door about 1/3 of the way from the back. This doesn't leave much room for a slide on the curb side.
As for motorhomes, most of the diesel pushers have only one door and it is usually right in the very front of the the MH so there is room to put the dinette and a slide there and also put a large slide on the street side.
The gas powered MH usually have a door located somewhat like a TT and so they have the dinette & slide usually located on the street side like a TT.
All this is just a guess based on my observations.
Barney