Mechanical complexity. A collective pitch helicopter requires a bunch of expensive moving parts and linkages to run the swash plate. The most traditional design

Mechanical complexity. A collective pitch helicopter requires a bunch of expensive moving parts and linkages to run the swash plate. The most traditional design requires 3 relatively powerful linear actuators of some sort as well as a spherical bearing at the swash plate, then 2 linkages to the rotor head and 2 more bearings on the rotor head itself. A quadcopter requires brushless motors (usually with 2-3 typical sealed ball bearings each) and props. Less moving parts, broadly generalizing, means fewer ways to fail.