Drones are controlled by varying the speed of the motors while helicopters are controlled by varying the pitch of the blades at various points in the rotation. If a motor fails on a drone then it’s lost, there is no way of controlling it. The helicopter rotor will spin while falling and the pilot can then mechanically control the pitch of the blades purely mechanically which gives some control.
The change of pitch in the helicopter rotor throughout the rotation can be seen here https://www.youtube.com/watch?v=0shLv2_Rtnc
Maybe they’re not able to use stairs