why did we start with main+tail rotor helicopters? Nowadays 4-rotor quadcopters appeared and have taken over

The quadcopter advantage is in it's mechanical simplicity, but having fewer longer blades is better for aerodynamic efficiency(at the expense of max airspeed). This matters less at RC scale, but at full size, even an all-electric helicopter will likely have more range than a quadcopter with the same batteries.

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.

Upgrade that 'maybe' to 'definitely'. The disc loading on a helicopter is typically much lower than on a quad (excepting crazy designs like the giant human-powered quad) which makes it significantly more efficient for the same overall size.