When a police officer comes up on someone they have zero idea of their history, mental condition, or any developmental disabilities.
And they have to figure that out right then, on the spot, in the street- often when that person is in the middle of a breakdown or other episode and actively trying to harm themselves, others or the officers.
That is something a fully qualified mental health professional often can't do on the fly in an instant under stress, but we expect cops to do it.
If there was a flag on a persons ID or license number that allowed the dispatchers to give a heads up to their condition, or a flag on an address for their home so that dispatchers can give a heads up when sending offocers, that would give responding officers a whole lot more info to base their actions and decisions on.
But, people will reject it based on the idea they don't want thier loved ones "labeled" or "in the system"- so if it ever comes down to police getting called they will be working on much less info on how to deal with the person than they could be, so the odds of a bad outcome rise considerably.