My main reason would be "too tired" since I have two young kids (< 3 y.o.).

Weather is the other big reason (too hot, too cold). I don't have easy access to a shower at work, so if I arrive sopping in sweat or rain, the best I can do is use a towel by the sink and put on fresh deodorant. (Ack.) And I'm not tough when it comes to cold weather. Anything less than about 45F is too darn cold especially if there's no morning sun.