Pizza is bad for you because pizza is high in saturated fat and calories as well as sodium and carbohydrates.
The high saturated fat and calories is from the cheese and processed meats.
Pizza is also often loaded with heavy toppings which makes it a calorie dense food and has limited nutritional value when the pizza is not made with healthier ingredients.
Many pizza toppings like sausage and pepperoni are high in saturated fat which can raise your cholesterol levels.
Pizza dough and even processed cheese often contains high levels of sodium, which can also raise your blood pressure.
And most pizza crusts are made from white flour which is a refined carbohydrate that has little nutritional value.
A single slice of pizza that has added cheese and meat toppings can be very high in calories and contribute to weight gain as well.
Pizza can be healthy when eaten in moderation but just don't eat too much.
As long as you're eating other healthier foods and a well balanced diet the pizza is okay to enjoy once in awhile but it shouldn't be your main food.