Why are there more stomata on the bottoms of leaves?

This is because, to prevent water loss. If the stomata were on the upper side of the leaf, it would be in direct contact with the sun, which would absorb all the water from the leaf, when the stomata is open (which is around 9am and 3pm). This would not be essential for the leaf, as it would not be able to photosynthesise, hence, unable to respire, therefore, would die due to a lack of food and energy.