Where women are concerned, we also have to take in to account that the gym is still considered very much a male domain and some women still feel uncomfortable with the metal weight style set-up, even if done at home, it's just not a female domain...much as yoga or step classes would be a "No way!" automatic thought for most men whereas women would be jumping all over themselves with excitement to get to a class (and the DVD set-up, as Jim mentions)...if you know what I mean.
Give a woman a KB (even though it's an old piece of training equipment, newly packaged) and she will more than likely stick with it...stand her in front of a barbell and you may never see her again.
The other aspect, as mentioned by Henry is that not everyone has a passion for exercise; in fact if you consider who exercises and who has problems sticking to plans, how many of those would have been active as youngsters and how many would have been the sort that didn't like sport/outdoor playing etc whilst they were younger. Is the carryover easier, mentally, for those that enjoyed being active when younger? My parents weren't active, no sports whatsoever, but I recall doing yoga, squash, jogging, football (soccer), judo, Tae Kwon Do, along with small amounts of gymnastics and other stuff.
Where the time is concerned, rarely is it the true issue because people that want to do something will always find the time, whenever possible. And for those that keep having an exercise session of 15-30 minutes whittled down, guaranteed time is not the underlying issue there either, they just aren't telling you the real reason for not wanting to do the routine.
36.5 kg /80.3 lb Middle-Finger DL (right hand)...