I suspect a lot of it is market differentiation. From a sheer practical sense the difference between an all electric and one that only has a gas cook-top and, perhaps, a gas/electric fridge is minimal.
Marketing is what drives much of coach design and when something "innovative" resonates with the consumers for whatever reason that drives more of the same. MH design is not immune to current fads and the associated bragging rights.
Have you ever seen the Marathon (I believe) that tried the slide out patio...that one didn't take off but if it had you would see others jump on the bandwagon practical or not.