Maybe not well articulated, but Alo is basically correct (if I am reading him correctly).

On rim brakes, since the brake pads need to cover more distance before hitting the rim, the mechanical advantage of the lever over the pads needs to be less less (so that more pad travel distance can be covered as you pull the lever).

Part of the reason that disc allows more mechanical advantage of the levers over the pads is that the pads can be set up very close to the rotor, so you can set up the system with the lever having more mechanical advantage over the pads without spending a whole bunch of lever throw just reaching the rotor.

Not sure If I explained that any better, but there you go.