When the big rotor does not get enough quick hot for efficient braking so you remove all the material to get it heating fast enough to get efficient but loosing dissipation of heat bonus from big rotor. bigger and thicker rotor with less material to dissipate heat
Or when marketing is more important than Physic's.
Society of who get the biggest
the pinacle of stupid marketing
the type of pads we use work fine when cold.............unlike carbon or race pads used in performance/racing cars they do not require heat to work. We do need the rotor to dissipate heat especially on long downhill descents though.