Road bike helmets tend to be light weight and have alot more vents so they're cooler. The higher end road bike helmets are also aerodynamically designed to lower drag in the slip stream.

MTB helmets tend to have fewer vents, so less of the surface of the head is exposed to poke hazards like tree branches, and they are also heavier and of course hotter to wear.

My MTB helmet is supposed to be a "Hi-Vent" design and still has nowhere close to the number of vents as a well vented road bike helmet.