Because it's a difficult motion to do without some sort of support or axis supporting the center of the wheel. With a normal wheel, there are two ways it can more: clockwise and counterclockwise. With the Wii wheel, it can also rotate, yaw, rock, twist, and every other type of movement you can think of, just because it's not anchored to a steering column of any sort.

Also, it's not very precise.

It's only redeeming factor is that it can supply some novelty value if you're playing a wacky game like ExciteTrucks or something.