Because there’s twice as much to heat. It’s the same with traditional ovens…the more food you have in the oven at the same time…the longer it will take to bake. You have more food pulling the heat, so to speak. Did I make sense? haha
Microwaves work by causing food to absorb electromagnetic energy from the microwave itself. Because there is only X amount of energy available, doubling the volume of food doubles the time taken to reach the same temperature.