From 1 May 2014, all pilots in Cat. 1 events must wear at all times while flying a helmet certified to either EN966 (airsports), ASTM2040 or SNELL rs98 (snow sports). For hang gliding, EN1077-A and B (snow sports) are also allowed. Before that, wearing EN966 certified helmets has been mandatory in Cat. 1 events since 1 January 2010.
That was decided to encourage a broader choice of models and sizes at lower costs, and make more available the protection by helmets fitted well and securely, and replaced regularly.
Moreover, recent research has shown that a significant proportion of the brain damage from impacts is from rotational forces. Technologies such as MIPS are available in snow sports.