Rarity?

Drum brake systems are usually lighter than discs.

Not if we compare on a fair, like-to-like basis (equal stopping power). To approximate the stopping and heat-shedding capacity of a disc brake system, you need a much larger/heavier drum brake system. Sure, A-body 9" drum brakes weigh less than A-body disc brakes...but with the discs you don't have to pray as often or as hard.