A quadrotor (or 8-rotor here) drone depends on rapidly throttling the propeller speed on each arm up and down, something that you can't do with a mechanical transmission without adding a lot of weight. You can do a gas/electric hybrid, but again, weight.
I kind of also wonder why not a more traditional articulated rotor helicopter design. The point of the quadrotors is they're cheap and simple, despite being much less efficient. Surely the capital investment in a full fleet of these things would favor a true helicopter rotor.
