Why do SRAM RED and Force eTap AXS rear derailleurs use the new Orbit fluid clutch instead of the Roller Bearing Clutch used on SRAM’s mountain bike rear derailleurs?

Road chain management requirements are different than MTB, so they do not need to use the same mechanism. Our Orbit™ fluid damper is speed sensitive so that cage movement resulting from gear changes do not activate the clutch mechanism, only high speed cage rotations created by bumps pulling down on the chain will. That way the clutch is only used when it is required.