I think Trevor might be right about lack of motivation or commitment...at least where my DH is concerned. He works out on his own at the gym, but not with the intensity that is required for Cathe's tapes, or any workout that requires planning and mental focus.
I watch him do bicep curls, for example, and he just kind of quickly throws the weight around. I tell him how much I love S&H, how lifting weights with purpose and concentration has really made a difference for me, but it's like talking to a wall. He's watched a couple of Cathe tapes before, and admits they look hard, she's in good shape, etc, etc. However, there's no talking him into using one.
Why would someone want to waste an hour working out w/ poor form? If you're going to put in the time,it should be productive, IMHO.