For the same reason a car engine’s RPMs will drop when you start going up a hill (unless you increase the throttle).
Spinning up the generators, and keeping them spinning, takes energy on top of whatever the load is. If the load increases, but the input energy does not, the difference is robbed from the momentum of the spinning mass. This slows the generators down thus the frequency drops.
Spinning up the generators, and keeping them spinning, takes energy on top of whatever the load is. If the load increases, but the input energy does not, the difference is robbed from the momentum of the spinning mass. This slows the generators down thus the frequency drops.