Unlikely he landed directly on his head. More likely his helmet hit the ground as a secondary impact after his body. He would have hit the pavement with a velocity of about 35 mph. Hitting a non yielding surface at that speed would often be lethal. This is entirely different than crashing on a motorcycle at speed and sliding down the road. He went from 35 mph to zero in a fraction of a second. The sudden deceleration is what kills.