The emphasis is on the police handgun only because it is the one most readily available. For serious business, the shotgun still holds an edge. The upland bird hunter has his pump action shotgun. But there is another side to shotguns that provides practical service in many more applications. The short barrelled shotgun was used by military forces in both World Wars.
Most soldiers are armed with rifles.
Police officers are armed with pistols. The sidearm is convenient. It is always present. When suddenly confronted with a threat, the police officer can quickly draw and employ his defensive weapon. Yes, for military or police, the handgun is a defensive weapon.