The problem lies with legal conflicts. "Drone guns" are essentially frequency jammers, which under FCC regulations are illegal within the U.S. The deployment of jammers requires current FCC code be modified in a manner that allows some to employ jammers while denying others to make use of the technology. In a way the need to change FCC code was rendered pretty much moot during the passage of the defense bill that reinstated drone registration, and passage of Part 349 provided permission for government agencies to down remotely piloted aircraft.
If anyone really wants to see how effective frequency jammers can be, visit a military training range when they are flying old EW equipped A-6 Intruders or other electronic warfare type aircraft. They can turn the lights off in a city. Our military has a vehicle mounted device that can scramble pretty much any frequency at a range of several miles, and they can be used in either a directional or omnidirectional mode. They devastate the 2.4GHz and 5.8GHz frequency bands.