I think it's because 5ers are used a lot for full-timing and full-time folks put more emphasis on the living area than the eating area. A lot don't really want a dinette (or table and chairs) at all. They'd rather use the recliner or couch and tv trays.
Also, with the couch/recliners in the back, it is easier to put the TV in a position that is more friendly for viewing.
And, if the living area is on the side, then it will look directly at the kitchen and I suspect folks don't want to spend their days looking at the kitchen - and if they look out the window, all they see is their neighbor.
There is probably more room to flip the recliners around in a rear living than in a side living in order to look out the window - and again, the view to the rear will likely be better.
Personally, I love my clipper's rear kitchen with the door right there. But, I have a picture window on each side plus one in the rear and the windshield in the front, so I have a view no matter where it is.
