As Humans, we are herd animals. Herd animals protect themselves from predators by hiding in the crowd. If you are in a herd, and you see a lion, you don't have to run faster than a lion, you just have to run faster than the next guy. In the long run, the entire herd benefits because the slower ones get selected out. Overtime, we have established social norms that encourage herd behavior, and discourage going against the flow.
Here is where this type of Social Proof CONVERTS your visitors into contest participants, and contest participants into buyers!>