I feel compelled to link to Jonathan Edwards' famous resolutions, but that isn't what this post is about.
For me, the really obvious reason why people so rarely keep New Year's Resolutions is procrastination. If they're willing to put off deciding to quit smoking or whatever until the New Year, then chances are they're going to be willing to put it off another year. If you really want to change something, you should start it when you think of it. I took up jogging, and I did that a week and a half ago. Why? Because that was when I got sensible shoes for it.
I guess the exception is people who only ever introspect and try to figure out what needs changing once a year, on New Year's Eve. But then, chances are they're so unused to it, they don't introspect deeply enough.