Is the increased prevalence of asthma and allergy in the younger generations because parents kept them too clean?

Not exactly. One factor at home can be that we drop our sandwich in the floor (or dirt) and immediately throw it out. In years past we would dust if off and eat it anyway. Such exposure to micro-organisms stimulates the immune system. The hypothesis suggests less of these exposures(going overboard with purel etc) may prevent the immune system from "getting a workout" and being as strong.
The Hygiene... ...Hypothesis is based on findings that people who had fewer life-threatening infections and less exposure to intestinal parasites have more asthma and allergy; it does not actually refer to how clean our parents kept our homes.