Motocross (MX) helmets simply have more safety features, such as added padding and EPS liner. That's because MX is a more dangerous sport and could send someone flying at a bigger impact compared to an MTB crash. If you compare MX and MTB helmets side by side, you'll see that MX helmets typically have more material. Moreover, MTB helmets can sometimes be a little bulkier in terms of design since someone who rides a mountain bike won't go as fast as a motocross rider would.