Apparently, high battery density enabled electric propulsion, and electric propulsion enabled multirotor designs (as a tiny DC motors can be placed individually at each rotor, eliminating the need for a heavy, cumbersome transmission).

And multirotor offers numerous advantages: design simplicity (software applying differential thrust to rotors instead of many complex mechanical parts in a helicopter), cheaper maintenance, quieter profile, VTOL operation using wings in level flight further reducing energy/noise, and the ability to survive the loss of a rotor or two. All of this makes it a true improvement for urban transportation.