From what I can remember, the overseeing bodies (whatever they are) are not convinced that requiring helmets would reduce serious incident rates, and in fact convinced that this would decrease overall bike ridership.

I'd speculate that the metric of "injuries per kilometer cycled" wouldn't budge because of a helmets requirement.

Can't find a good summary of this now, but some bits of this are googleable.

Some sources:

[1]: "Cycling UK wants to keep helmets an optional choice. Forcing - or strongly encouraging - people to wear helmets deters people from cycling and undermines the public health benefits of cycling. This campaign seeks to educate policy makers and block misguided attempts at legislation."

[2]: "Even if helmets are 85% effective (and assuming q = 0.5 as above), the number of cyclists’ lives saved will still be outnumbered by deaths to non-cyclists if there is a reduction in cycle use of more than 2%"

[3]: "Enforced helmet laws and helmet promotion have consistently caused substantial reductions in cycle use (30-40% in Perth, Western Australia). Although they have also increased the proportion of the remaining cyclists who wear helmets, the safety of these cyclists has not improved relative to other road user groups (for example, in New Zealand).

The resulting loss of cycling’s health benefits alone (that is, before taking account of its environmental, economic and societal benefits) is very much greater than any possible injury prevention benefit."

[...]

"Evidence also suggests that even the voluntary promotion of helmet wearing may reduce cycle use."

[...]

"Even with very optimistic assumptions as to the efficacy of helmets, relatively minor reductions in cycling on account of a helmet law are sufficient to cancel out, in population average terms, all head injury health benefits."

[4]: "With 290 cyclist fatalities in 2022, cyclists were the largest group of road casualties. Of these, most were killed by collision with a vehicle (206 bicycle deaths)."

[5]: "Cycling levels in the Netherlands have substantial population-level health benefits: about 6500 deaths are prevented annually, and Dutch people have half-a-year-longer life expectancy. These large population-level health benefits translate into economic benefits of €19 billion per year, which represents more than 3% of the Dutch gross domestic product between 2010 and 2013.3.

The 6500 deaths that are prevented annually as a result of cycling becomes even more impressive when compared with the population health effects of other preventive measures. In an overview, Mackenbach et al.11 showed that the 22 new preventive interventions that have been introduced in the Netherlands between 1970 and 2010 (e.g., tobacco control, population-based screening for cancer, and road safety measures) altogether prevent about 16 000 deaths per year.

Still, our results are likely to be an underestimation of the true total health and economic benefits."

[6]: "Riding a bicycle to work every day reduces the risk of premature death by 41% (risk of dying from heart disease: -52%; risk of dying from cancer: -40%)."

[...]

"Regular cycling boosts physical fitness and compares to 1 to 2 weekly gym sessions."

[...]

"Bicycle use not only improves physical health, but also has a positive impact on mental health and subjective well-being."

[1]: https://www.cyclinguk.org/campaign/cycle-helmets-evidence

[2]: https://www.cyclehelmets.org/1249.html

[3]: https://www.cyclinguk.org/article/why-should-highway-codes-a...

[4]: https://english.kimnet.nl/publications/publications/2023/11/...

[5]: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4504332/

[6]: https://www.government.nl/binaries/government/documenten/rep...