Because it’s an added inconvenience? Because they don’t like their hair getting messed up (there was an even greater drop in teen girls cycling in AU after helmet laws)? Regardless of how you feel, it is a plain fact that helmet laws reduce cycling rates.
Fact: The safest place for cycling is the Netherlands. Fact: Few cyclists in the Netherlands wear helmets (amongst adults, generally they’re either sports cyclists or foreigners). Fact: Helmets thus are NOT a prerequisite for safe cycling. Fact: Countries with helmet fetishes (UK, USA, AU) have worse cyclist safety. Fact: Mandatory helmets laws *reduce* the amount of cycling done. Australian states are now looking at relaxing helmet laws, because of the damage done to cycling participation. If you want safe, mass cycling, the facts *clearly* show that you should follow the example of the Netherlands - NOT the failed example of Australia, the nirvana of the helmet lobby.