A desire to comply with the law and fear of fines no doubt had much to do with that trend, as did publicity about the potentially life-saving benefits of buckling up. Motorists who developed the habit as children copying their parents may not give any thought to the legal requirement, let alone harbor moral objections to it.
To begin with, police in states that require adults to use helmets have the authority to stop a motorcyclist simply for failing to wear one, while police in most states still need some other reason to stop a motorist before they can cite him for not buckling up. Even in states with primary seat belt enforcement, a helmetless motorcyclist is more conspicuous than an unbuckled motorist, making him more vulnerable to traffic stops.
Another reason helmet laws provoke more resistance than seat belt laws is the comfort factor: While buckling up is relatively painless for most people, wearing a motorcycle helmet that weighs a few pounds and covers most of your head can be tiring and sweaty.