I see at least four reasons this is the reasonable policy - Bicyclists:

* have a much better communication with their surroundings: unimpeded sight lines, and typically less sheltered from surrounding sounds

* suffer more risk and do less damage in a collision

* lose momentum in stopping that must be spent getting back up to speed - as to the significance of that last point, it affects the speed at which you arrive and the exertion you expend doing so - you can look to the preferential user experience of electric bikes as an example of how different cost/benefit energy/velocity trade-off influence use patterns

Finally, it is in society's interest to incentivize bicycles over cars, as they exhibit far fewer costs on society, in the form of parking, pollution, and physical violence.