I think the time aspect is really important and, on this site at least, talking to the converted.
But with a young family or even an older family but long work hours, I can see how difficult it could be.
I do still try and commute on my bike, and walk as much as I can but its not easy to fit everything in. Walking is easier but just takes longer (although by the time you lock a bike up etc, sometimes you may just have walked it)