Why did some states change their laws for the requirements of motorcycle helmets?

A few states changed their laws from being required to wear a helmet while operating a motorcycle to being required to wear a helmet when what is usually under 18 years of age. Why?
What influenced their decision? Was it because they simply didn't want to wear a helmet? What about unintended concequences?
Also, why would they remove the helmet laws for adults but leave the requirement for minors? Was there a specific reason?