Interesting. I live is a small town in a very rural area and the police here have been patrolling with “assault weapons”(in this case they are really REALLY assault weapons) on board for decades. They keep M4s or MP5s in the trunk of their patrol cars. I’m a little surprised that they are just now getting around to doing this is a place like Miami.
I am guessing political reasons. The Police chief and Mayor would have to justify the police officers using their weapons to the news media and the public.
Probably in rural areas the citizens and the police have a much better relationship. The citizens have respect for and trust the police, and expect them to be armed well enough to do whatever they might be called on to do, and assume that they are intelligent and well trained and will do the right thing.
I would guess in the bigger cities you have more of the welfare state anti-police mentality, plus the people and the lawyers looking to hit the legal lottery in a lawsuit against the police.
I am sure the lawyers and the families of the thugs would be screaming that the police are mowing down "disadvantaged urban youth" - or however they would be defending the thugs as - and it had to finally get pretty bad to be able to allow the street cops to have a patrol rifle available to them in order to give them a chance against the thugs armed with AK-47s.