The limited bandwidth of practical microwaves shared by everyone in the footprint of a satellite, which is thousands of square kilometres. More satellites help, but since it hears the signals from every person on earth in its footprint, even if that person is connecting to a different satellite, there are limited gains when you reach the point where they have a lot of overlap - literally limited by geometry. Compare that with fiber, which allows for virtually unlimited unshared service bandwidth that can get faster as it’s built out and becomes more popular.