Because in the US, states get to make some rules, and the federal government gets to make some other rules, and each is protective of their rights and prerogatives as enumerated in the US constitution, as amended. Look into amendments 9, 10, 13, & 14 for more.

When the US federal government has wanted to enforce things like a nation-wide speed limit, or vehicle safety requirements, they have had to threaten to withhold highway funding to get states to go along by passing state-by-state laws. In theory, the same could be done with helmet laws, but enough political energy has not materialized for that to happen. This pattern will have implications for full-self-driving vehicles.