4 rotors is also a much more difficult “controls” problem. Helicopters actually glide reasonably well if their engine cuts off. They’re not super insanely unstable.

Quadcopters are basically obligate fly-by-wire vehicles which necessitate at least pneumatic PID controllers if not computer chips to fly.

You can make a helicopter work reasonably well a lot earlier in a pre-computer “tech tree” than you can a quadcopter.