Imagine a spinning rotor blade, as the blade moves through the air, it pushes air down, lowering the pressure above the rotor. At the tip, the high presssure seeks low pressure above the blade and produces a wing tip vortex. The flow at the tip rolls up then around, then down, effectively pushing down(downforce) on the rotor blade tip, instead of pushing up(lift). But that's not all, the rotor is spinning so the next blade is going to interact with that vortex. The spinning vortex is still circulating in the same direction, down away from the rotor, then up then reversing towards the rotor, and finally flowing downwards. That means the next rotor is running into air that's going downwards which in practice, reduces the effective angle of attack of the rotor blade. That means that in order to get the ideal lift means you need a greater angle of attack, which increases drag.