Because they set the goal too high. Take smoking cessation as a resolution. That's their goal so they spend the whole year, or month lol, trying to quit cold turkey over and over again. They might be more successful if they set smaller goals, like, by the end of January I won't smoke when I drive any more, and by the end of Feb I won't smoke while I talk on the phone or drive, and by March you'll only smoke half a cigarette, etc., until by the end of the year they could quit, making their resolution. I wrote a hub about this.