PS/2 keyboard/mouse are interrupt driven which saves power on laptops. While USB ones are polling based which require controllers running all the time.
Good point. On a video recording system we had problems with dropped frames from the USB camera. When an external USB keyboard was connected the problems became rarer. Obviously the keyboard kept some component to enter deeper energy states, and when everything staid up all the time frames were no longer lost as commonly.
Surprising to me as a layperson in this field is that camera with 30 fps would allow entering a deeper state than a keyboards with a data rate orders of magnitude smaller.
