Because they are not real resolutions. Most ‘New Year’s Resolutions’ are vague or complex wishes of what you would like to change. A wish: I want to lose weight, I want to stop smoking, I want to be a better person—is not a resolution. Enthusiastic attempts to fulfill these so called resolutions usually cease after the first couple of months of the year and then all is back to normal.
If you really want to change something, you need tangible targets:I want to lose weight. How can I realistically go about this? Make a food diary, stop taking seconds, how can some more exercise fit into my normal life? Is there a walking club in my area I could join?
Make it feasible. Make a plan that fits into your life. If you want to change your life take it step by step.