From my understanding, the biggest reason was that most buildings were not wheel chair accessible until after the late 80's. Once this started to be mandated, wheeled luggage became useful.
Good point, but I'm not sure this is a major reason. I've used wheeled luggage over long walking trips, and it's still a huge improvement, even if you have to lift it over some steps here and there. I would often wheel luggage from my place in SF to the Powell station, and you had to pick it up for a few short staircases but it was still great.
