Why do we seldom see small unmanned aircraft with gasoline engines?
Because electric is the best propulsion method available for small commercial UAVs and their mission profiles:

-Significantly quieter

-No smells involved

-No flammable liquid involved

-Way easier to operate: properly tuning an RC gas engine is not for everybody

-Much lower vibration and on frequencies easier to filter out

-More controllability, a BLDC (brushless DC) motor has near-instant response to the throttle and thus enables the actual, simple building scheme of quadcopters