Why are more stomata present on the lower surface of the leaf?

More stomata are on the lower surface since they are shaded and more protected from drying 
winds. This results in lower transpiration rates. 

(Stomata on the upper surface may be more susceptible to entry by disease-
causing microbes. They may get clogged more easily by water and dust. Also stomata on the
upper surface may allow more mineral nutrients to be washed out of the leaf during heavy rains.)