Good capture of the rainbow, but since it's not really a defined object and rather blurry by nature, the photo feels like it lacks focus. Maybe something in the foreground would have helped this.
It's a nice enough photo, but without some sort of contrast it really has no place in this contest. If you hadn't put your reasoning in the title I'd have really been confused about this photo in this contest. Perhaps a wilted plant next to an empty watering can or one filled with dirt rather than water.