Because it must be the right size to handle the CFM airflow of the air handler blower. CFM is cubic feet per minute. ACs need roughly 400 CFM per ton. Undersized: There will be too much pressure in the ductwork, which will be noisy and might eventually cause it to come apart. Your system will work harder. And your rooms won’t get enough air to be as cool or as warm as you want them to be. Oversized: There will be too little air pressure in the ductwork to move the air, and it won’t get to the rooms, especially those that are furthest from the air handler. You will have a significant temperature imbalance from room to room in your house.