Try baking a pizza on the top rack of your oven and you just won't get that crispy bottom crust. Bake a tray of cookies on the bottom rack, and it's pretty likely you'll have burnt bottoms. But move the rack into the right place, and your food will be more likely to cook right, too.
Your pizza might not be browning because the oven temperature isn't high enough. Heat your oven to 500 degrees Fahrenheit to quickly cook and brown it. Pizza dough needs to be slightly chewy to support a bevy of toppings.