The pizza bottom can turn soggy when the size of the crust is too thick. Generally, the thicker crust soaks more moisture from the veggies and other toppings. And if you allow the pizza to cool down for a long after baking, you will get more softness at the bottom of the pizza base. If you want to avoid it, you can either select a thin pizza crust or add fewer veggies and other toppings with more water content.