I think the states should continue to decide if they want a helmet law or not.
However it would not surprise if eventually they all pass one. When I lived in California it upset a lot of people when the law passed. However like most things including wearing seat belts and no-smoking in buildings eventually it becomes the accepted norm. I can't imagine driving without a seat belt now.
The helment law is believed to protect bikers from possible brain damage or skull fractures in the event of an accident. There were a couple of high profile instances that took place in California fairly close together. Those accidents probably led to the passage of the law sooner rather than later.