Why haven't quadcopters been scaled up yet?
Wouldn't they be more stable and easier to control than helicopters?


No, they would not.

Quadcopters don't have any special inherent stability. When you increase power of one of the rotors to pitch, the increasing pitch will not do anything to the power difference and therefore the pitching moment.

The advantage of quadcopters is that the rotors can be fixed pitch while single (or double) rotor helicopter needs complex control mechanism. While this is huge advantage for the small scale devices where each rotor can be powered by its own simple electric motor, the complexity of either additional engines or long transmission shafts would outweigh any advantage from the simpler rotors in full-scale vehicle.

And why can't full-scale helicopters use electric motors like the small ones? The reason is that when you scale an airfoil up, the lift it produces increases with its area, which grows with the second power of size, but its weight increases with volume, which grows with the third power of size. Therefore models have much more lift for weight and can afford simple but relatively heavy batteries while full-size aircraft need propulsion systems with higher power density.

And then there is also the factor of safety. In case of power failure, helicopters can still glide to the ground and still land vertically using autorotation. But since the rotor rotating speed can't be changed without power, controlling the helicopter during such manoeuvre requires variable pitch rotor. So there goes the main advantage of quadcopters.