I think it's the same reason drunk driving went largely unpunished for so long. Every cop and DA knew that the jury would have people on it who had driven drunk. The defense would convince them that they could easily be as "unlucky" as his client and the jury would acquit.

Currently, nearly every person is a motorist and nearly every motorist breaks the law every time they drive. The cops, DA's and jury are all part of this scofflaw-driving tribe and are ready to forgive him/her for hitting the "other" who shouldn't have been riding on that road, at that time, on that day...

Maybe, if we ever get more people to ride, we can change this. However, I don't see us getting substantial numbers of folks onto bikes until people see cycling as a safe means of travel, and that will require a substantial investment in law enforcement that is just not on the horizon at this time.