I suppose lack of sales/popularity of the DW attachment could also have been from other issues. How much did the dish attachment cost? If enough, people might "think about it for one day." And then never get around to it... Or else just decide to wait until the baby in diapers is big enough to wash dishes, thus saving the cost of any mechanical gadget. (Even in the 1980s, when I grew up, there were apparently parents who took that attitude: Why spend $$$ at Sears for something the kids can do for $0?)
Then another factor might have been the kitchen space. Is it big enough for doing the laundry? Or should the washer go to its accustomed home home in the cellar? If in the cellar, it would be less than convenient for doing the dishes. (Trust me: this idea came up for me, once. A dishwasher is My Big Dream, and someone suggested it'd be easier to install it in the laundry room. He made a good point as far as space and plumbing issues, but all I could think of was the pain of the long trips carrying dishes out and back again.)