Maybe because it has to do with protecting the brain from trauma. Today there is an article in Los Angeles Times (front page) "War injury leads to advances at home; The military takes the lead in brain trauma research, giving hope to wounded civilians of a 'silent epidemic.'"

The present article discusses how efficient ancient helmets would have been in preventing trauma from both blunt and sharp weapons, and why some military personnel of the Viking period appear to have chosen not to wear metal head armor.