The reason that bike helmets are better for cycling.

One: Bikes move faster then stakers. It only takes a 12 mph head strick to kill you. On Stakes most people only run at 12 to 14 mph. on a bike it's more like 15 to 25 mph. So Bike helmets have to be able to take a blow form a ridder hiting the ground at 25 mph. So there are thicker and will take a lot more then a stake helmet will.