The riots of the sixties and increasingly militarized organized crime (drug cartels) are some of the reasons given for the militarization of the police since - turning them from members of neighborhoods and communities into pseudo-occupiers for a simplified answer.
The people themselves are no longer trusted to make decisions and take actions against criminals on their own. The left has successfully split the police from the people to the point that today most police consider the people they serve to be 'civilians' as opposed to themselves. 'Us' and 'Them', no longer 'we'.
I was talking with a photographer from a small town on the Yucatan peninsula. It’s not a hugely popular tourist site because it’s a natural harbor so it’s nice, but not like Playa del Carmen or Cancun. He said when he was growing up they had no police. If somebody started offending against the community, they would have neighborhood watches. If someone saw someone going into someones house, they would tell that someone. Then the guys would take the offender to a quiet place and tune him up.
Then the police came and, he said, when the police came i.e., the government comes and says they’re going to take care of a problem or a function for the people, they no longer feel responsible for handling it themselves. And then if the police don’t take care of it, the perp gets away with it.
So the moral of his story, which I will never forget, was bringing police made crime go up.