Why does this statute raise equal protection issues instead of substantive due process concerns?

Equal protection calls for state, to guarantee the same rights, priveleges, and protection for all the citizens. Substantive due process on the other hand calls upon court protection of certain fundamental rights from government interference, even if procedural protections are present or the rights are not specifically mentioned elsewhere in the US Constitution. The statute that was raised by the state conferred certain privileges on part of the motorists, requiring some to always wear helmets while others were not.