Young adults living alone or in group houses. You get your own phone. You are out most often and away from home. You make many of your chatty calls from work. No phone bill splitting, either, especially in groups houses.
Kids are gone, both adults work long hours and are always on the go.
Indeed, very little calling is done from home any way. You can always share the out going calls with your wireline when you are not on the computer. They just talk too much. But many families think about getting second and third lines for teens and for computers. A wireless phone might well substitute for one of those landlines.