[JohnBooty]

Surely it's because: on a motorcycle you usually only hurt yourself?

In a car you tend to hurt other people including any kids, etc that may be along for the ride.

[Causality1]

More likely it's just because cycles existed before the safety regulations and there would be too much backlash to banning them. Plenty of other things that only harm yourself, like drugs or unpasteurized milk are also illegal.