Why do electric versions of helicopters need four rotors, or: why are they quad copters?
Could it be that their motor do not have enough torque? Then again electric motors have much more torque than their piston counterparts, which early helicopters used.
Seeing that there plenty of such toys that have the traditional helicopter layout, there is no "must" involved here.
A typical toy helicopter (image source)
But quads and other multi-rotors have a few of great advantages: symmetry, commonality between the engines, no need for complex variable pitch of the rotors. This makes manufacturing cheaper, and the control algoritms much simpler, allowing for much more autonomy and self-control.
It also allows for some pretty impressive moves that a normal helicopter layout may struggle to replicate (although given the right pilot, it sure can give the quads a run for their money).
Also — as opposed to full scale helicopters — it is not a disaster if a drone fails; failure is an option. Hence the problem of keeping all rotors running flawlessly and perfectly synced — which is an absolute must for a full-sized multi-rotor carrying people, but also a major headache to achieve, often to the point of being prohibitive — is not a problem for the drones.
