The explanation lies in how the stomata works. They regulate any gasses that passes trough them. When the temperature is too hot, it will close to conserve water vapor but this will block CO2 coming in also. Therefor plants don't want to have their stomata closed all the time nor opened all the time. Sunlight hits the top surface of leaves. If many stomata were to be located on the top, the plant would loose much of its water and when the stomata closes to conserve water, it won't get as much CO2. Therefor for most plants, the stomata stay on the bottom where the temperature is a bit cooler. Also it is protected from the wind since wind can cause evaporation of water. I hope this Helps c:
