Given that dual or quad copter designs are so much easier to balance and control and are a fairly logical design, how on earth did helicopters as we know them become the standard vehicle design for VTOL until the recent boom in drones?
Quadcopters control their tilt by adjusting the rate at the which the rotors turn. Spinning up the rotors on one side causes the craft to tilt. If you scale that up to something big enough to carry a human, the rotors gain so much momentum that changing their speed to change the craft's tilt angle takes too long and the craft becomes unstable. Helicopters tilt by adjusting the blades so they have a different pitch angle on one side of the rotor disk than the other, while the actual turn rate of the rotor stays the same. This is more complex, but not limited by increasing size like the quadcopter system, resulting in a more practical control system for large craft.