When soldiers encounter resistance, they get on the horn and call in the artillery or an air strike and level the place. Easy.
When police encounter resistance, they are required to negotiate, preserve evidence, and arrest the perps, alive, for trial. Citizens demand strict limits to what cops may do to them. Professional police forces prevent rioting by maintaining communications with all segments of their town/city and try to head off trouble by appealing to various community leaders to cool it for a while. This works best if someone of the force has a personal relationship with community leadership. Such as they both did high school together.
All this is hard.
When we offer the cops the opportunity to turn into soldiers, they are all in favor. Blowing the perps away is SO much easier than taking 'em alive. So handing out surplus automatic weapons, body armor, grenades, helicopters and armored vehicles to police forces encourages the cops to turn into soldiers.