It is like one helmet safety site asked:

If helmets are that important to our safety, then why aren't they required in for auto drivers who suffered far more head injuries than cyclists?

Also, the optimum helmet, according to a study in 1990, would have a clear face shield. Why hasn't that become the standard? Most head injuries for cyclists are facial.

A cycling helmet is designed to be effective at a speed...basically that of a person running. Why aren't runners required to wear helmets?