I think it's because you need to start somewhere and give yourself enough time to make it. Most people feel like they can start fresh with the new year, it's easier this way. You stop feeling bad for the things you didn't manage and just start again. Nobody says you can't start with your resolutions as soon as they come into your mind, just do it.
My resolution for this year were: get a grip on your depressions without meds, done, go to the gym regularly, done, finish the dam re-writes, not done, start a new multi-chapter novel, not done, full-time teacher Yes?No?, not done.
This year? All the not done, I finally want to play 'Für Elise' right (me and her just can't get it on) and I want to do something unexpected just because I can.