I think people mostly break the law "when little harm could come to them", and care slightly less about others. For bicycles, this isn't a problem because they

I think people mostly break the law "when little harm could come to them", and care slightly less about others. For bicycles, this isn't a problem because they pose little risk to others and most of the potential harm would be for the cyclist. So they only break the law when it's safe to do so. Motorists, however, are safe, encaged in their steel cage with seat belts, air bags and crumple zones, so they're generally safe. Most of the risks of their law breaking goes to other people, and if they care slightly less about the safety of others, that means their law breaking poses a much more serious risk.