A fair amount of people would like riders to wear helmets every time, every ride. A lot of people don't. It seems to me, if you want to change that, the best way to start would be to figure out why people are not wearing helmets right now. Which is the approach public health researchers use when trying to find out why people don't use clean needles. (Relevant part at 3:20, but the whole video is awesome. Bonus: subtitles in 31 languages. Learn how to sound smart about public health is Czech!)
While drug use and HIV transmission are much more serious than helmet use (and not wearing a helmet is not in the league as HIV transmission), the methods they used have a broad application. For those who can't watch the video, the researchers went to various cities in Indonesia and went to popular hang-outs of heroin addicts (or smackheads, as she calls them) and asked them a bunch of questions.
Question 1: Do you know how you get HIV? Almost everyone said, yeah, by sharing needles.
Question 2: Do you know where to get clean needles? Again, the answers are almost universally yes.
Question 3: Are you carrying clean needles? 1 in 4 or less say yes, and less than 1 in 10 use clean needles every time they inject heroin.
At this point, the equivalent tactic of helmet fanatics would be to start yelling and screaming about health costs and don't you love your family and think of THE CHILDREN and you'll deserve it when you die, you junkie. Instead, the researchers did something radical and outrageous and unheard of and asked the people why they weren't using clean needles and listened to their answers. And it turns out they had a very good reason: carrying needles was against the law. If the police caught you with a needle, you'd go to jail and possibly get HIV or killed there. So, for these heroin users, sharing needles was actually the less risky choice.
So let's extrapolate this to helmets. I'm guessing if someone with a background in statistics were to stand around at a horse expo or fair anywhere in the country asked people 1) Do you know the function of helmets, 2) Do you know where to get a helmet, 3) Do you wear a helmet, we'd learn some interesting things. I'm sure pretty much everyone would know where to get a helmet (most feed stores and even Fleet Farm carry them) and what they're for, but maybe half or less would actually be wearing them. So, a good thing to know would be why.
There a common theme among helmet evangelists that nothing is less comfortable than a TBI, so nothing excuses not wearing a helmet. Which sounds like "Nothing tastes as good as skinny feels," which I can tell you ain't true. Chipotle burritos, nuf said. And helmets can be EXTREMELY uncomfortable, especially if you're like me and have an oval shaped head, it's hard to find a helmet that fits and is comfortable. For years, I hated wearing helmets because they gave me headaches. It wasn't until the local tack store started carrying Charles Owens and GPAs that I figured out that helmets weren't the problem. The problem was I'd only worn Troxels and they are not very comfortable for me. Nothing wrong with them, they just don't fit me well. But most places around here only carry Troxels, so I was out of luck. When I actually got to try on multiple brands and styles, the two that fit be best were the Charles Owens GR8 and the GPA Speed Air. Both of which are expensive and not found at a lot of brick and mortar stores around here. Now imagine someone like me, but who doesn't live within driving distance of a store with many different brands of helmets. Either they order dozens of helmets from online retailers and hope they eventually find a comfortable one, or they might just forgo the helmet thing altogether.
And not wearing a helmet doesn't guarantee you'll die of a TBI, believe it or not. Anyone can point to tons of people who have ridden without a helmet - multiple times even! - and lived to tell the tale. There are people who have been riding a long time helmet-less and are still alive. So the implication that no helmet = YOU WILL DIE AND IT WILL BE YOUR FAULT is very very false. A helmet lowers your risk of a TBI, sure, and no one has ever said otherwise, but a lack of helmet does not increase your chance of an accident occurring.
And there could be plenty of reasons why people don't wear helmets. But berating them is not going to change anything. People will wear helmets when the advantages outweigh the drawbacks. How that happens is different for everyone. The USEF recently changed the dressage rules so that everyone has to wear a helmet in dressage, even in the FEI levels. But not if you're competing in an event under FEI jurisdiction, which only adds to the confusion. At Ye Olde Locale Showe, helmets for all. At Big Important International Event On US Soil, helmets for some, odd hats for others. It's just so weird.
But first, if you want people to wear helmets, ask them why they are not. And listen, instead of moralizing or berating. The answers may surprise you.