I'd think the reason is simple - car runs red light, you risk getting a ticket and fine and increases to your insurance premium. Hit someone while you're doing it? Now you're guilty of manslaughter.

Bike runs a red light? No one (including cops) cares enough to do anything aside from people swearing at you or giving you the finger. A car hits you while you're running the red light? Odds are it's still more likely to be considered the car driver's fault, and many cyclists seem to think they are invincible until they actually get injured so they don't think twice about the risk.

I will admit I am guilty of intentionally running red lights on a bicycle. But I've never intentionally run a red light while driving - again, because of the aforementioned disincentives.