Here's my analysis then, Richard. Just for you.
IMO life is cheap in Thailand. They just don't care and aren't educated well enough on road safety. Many (wrongly) believe that wearing amulets around their necks will protect them anyway, so why bother with the helmet in the first place??
If someone can afford a Honda click, then they can afford a helmet from Tesco. Alright, granted, that helmet isn't going to be the best quality, but it is better than nothing...
I also think laziness is a factor. Many thais just can't be bothered putting a helmet on their children's heads or wearing one themselves.
Hope this is satisfactory for you, Richard. And if it's not well too bad, I aint being paid to reply to you!