This sort of assumes that people put biking as highest priority in their daily activities which is rarely the case. People who slightly prefer biking are less likely to do so when it is slightly more inconvenient.
Part of effect is that helmet campaigns paint cycling as much more dangerous as it really is. So, more risk averse people are putt off bikes entirely and less likely to use them. It makes people associate bike with picture of accident instead of pleasure.
Part of effect is that kids cant use bikes on playground "normally" how they would without helmet - they cant chase, hop on bike, go 10 meters and hop down.
Part of effect is that you cant use bike as transport device when you need your hair to look reasonably good afterwards.
Part of effect is that helmet is one more thing you have to carry with you and it might make difference between "yeah kiddo take it" and "nah, it stays home".
As a teenager I never even bothered to get a helmet because I would have to ask parents to spend money on it, and it would be inconvenient to carry around all the time. It interfered with the flexibility and fun of a bike. If I were forced to use it (and God forbid, elbow and knee protectors), it'd probably cut my bike usage in half, because what was just a matter of hopping on and riding away would become a whole dress-up game.
I'm a cyclist. I race mountain bikes, used to race on pavement, and still avidly train several days/week on local roads and bike paths.
I wear a helmet when I'm training or traveling a long distance at speed. On roads with cars, higher speeds = higher risk for major accident.
I don't wear a helmet when I'm cruising to the pub for a drink. Yeah, I can still fall. But, I'm on the mountain bike (relatively easy to hop off, compared to a road race bike), at slower speeds, and generally not on roads with cars.
I do wonder if part of the problem is the type of bikes popular in some counties. The US has a large number of road race bikes relative to townie/dutch-style bikes. The townie/dutch bike is slower, lower to the ground, has a step through, and more upright seated position (better visibility). All should lead to fewer major accidents.
Part of effect is that helmet campaigns paint cycling as much more dangerous as it really is. So, more risk averse people are putt off bikes entirely and less likely to use them. It makes people associate bike with picture of accident instead of pleasure.
Part of effect is that kids cant use bikes on playground "normally" how they would without helmet - they cant chase, hop on bike, go 10 meters and hop down.
Part of effect is that you cant use bike as transport device when you need your hair to look reasonably good afterwards.
Part of effect is that helmet is one more thing you have to carry with you and it might make difference between "yeah kiddo take it" and "nah, it stays home".