Not exactly an ELI5, but here’s an ELI15. I used to be test engineer for one of the world’s largest helmet manufacturers. We made helmets for bike, snow, and moto. Ultimately, I was the one who understood the test standards and requirements for each type of helmet, and made sure we were passing those requirements with new models we developed.

The primary reason why different helmets exist for bike and snow is due to differences in the impacts that occur in both activities. Bike helmets are designed to meet the relevant test standards for bike helmets, which are based on real world scenarios involving typical cycling speeds and impact surfaces. For example, the CPSC standard for US/North America requires impacts on flat, curb, and hemispherical anvils, all of which imitate surfaces that might be impacted when crashing on a bike and at typical speeds seen when cycling. This standard also requires helmets to be tested in ambient, cold, hot, and wet conditions, to replicate different weather conditions and user choices.

The snow standard differs from this, and uses a different combination of speeds, anvils, and conditionings that are specific to snow sports. For example, there is no hot condition or curb anvil, but there is a penetration test where a metal spike is dropped on the helmet to imitate the impact of a broken tree branch or something similar. Different materials and construction are used to meet the different requirements for these standards.

A few other factors are head coverage, which is often defined by the test standard, and comfort. Snow helmets have full coverage and are designed to keep you warm, while bike helmets are generally designed to keep you cool and may sacrifice coverage for lighter weight and better aerodynamics.