A worthy question. My theory inside
In the east the sport of whitewater arose from hardboaters and the rafts came along later. Those who ran rivers were used to wearing their brainbuckets already. Western whitewater arose from bigwater expedition river running in larger boats and rubber rafts that shared more with fishing than slalom. Thus the brainbuckets were never really a part of the experience. I remember my first trips down the Middle Fork in the late 60s and early 70s we all wore hats to keep the sun off, none of us wore helmets, nor was the idea even mentioned.
I'd be willing to be wrong on this, it's just a guess on my part and somebody else might have a different opinion which I'd respect. But even Eliot DuBois who wrote the remarkable book "An Innocent on the Middle Fork" about his first descent of the river solo in a folbot kayak had headgear with him IIRC, but he was from the Northeast.