Why do we seldom see small unmanned aircraft with gasoline engines? Most commercially available FLYING WINGS (RC aircraft or UAVs) are battery powered. Why aren't there more with with gasoline engines?

PS: Sorry I edited. The question is specific about flying wings configuration.
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