Why can heating things in a microwave make them soggy?
Microwaves specifically heat water molecules in the food. This turns them to steam, and because the air in the microwave is actually cool, the steam then condenses. There is often not proper air circulation to move the steam away from the food. Often times the outside edges of the food will not be soggy, but rather burnt, because they receive more energy and the water can totally vaporize.

For better results when reheating food, do not reheat your food on "high" or the default power. Reheat it at a lower setting for longer time. For pizza specifically, I heat it in the microwave only to room temperature and then finish it in the toaster oven or in a pan.