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Disc brakes typically have more drag than drums. The springs in drum brakes pull the shoes back away from the drum surface. Obviously if your adjusters have spread them a bit too far they still drag on the drum surface, but their tendency is to not contact the drum.
Calipers have no mechanism to spread the pads away from the disc after brake actuation. Many caliper designs have o-rings on the slides in an attempt to prevent knock-back where rotation and flexing of the disc (or wheel bearing) will push the pads away from the disc. A lot of drag racers use drum brakes because it is easier to set them so that they will have no drag and leave braking performance to the 'chute.
Making a parking brake meet safety standards with discs is a pain in the rear. The force required to clamp a rotor and hold it is so high that it is only user-friendly with power-assisted hydraulics. Parking brakes can't be hydraulic because of the risk of fluid leakdown causing a park brake failure, but the mechanical force required is rough. My truck has 4-wheel disc brakes but the rear discs have a park brake drum surface on the inside rear of the rotor with the mechanical cylinder and shoes behind the rotor. It's 2 brakes in one, which is more costly (not to mention heavier, and forces large diameter rotors, thus large heavier wheels).
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