Momentum. Mass*velocity give you momentum, which is the reason the wheel resists changes to movement. Thus, once you get rolling, the bike is hard to tip over. Heavier wheels have more momentum, which is why they're easy to keep spinning once they get up to speeds. It's also why light wheels accelerate faster -- because there is less momentum.

Experiment: take off the front wheel and hold it by the skewer in both hands. Spin the wheel really fast and try to move the wheel from vertical to horizontal (90 degrees). It's that "force."