So to start with an exaggerated example, let's wonder why we don't want the control loop frequency to be 10x greater than the switching frequency. Assuming that we fix the frequency of switching (just for a simple converter, imagine this scenario:
Our buck switch has turned on, and turned off, and we are in a continuous mode where current is flowing through the inductor and the diode in a typical buck converter. Suddenly (and magically) we detect that our output voltage has dropped, and needs to be compensated. If your only "knob" to turn is PWM duty cycle, you've got just about a full switching period before you can begin to address the error signal.