And there's a tendency among pundits and observers to act like this is all being hashed out in a smoke filled room.
That isn't how any of this works. There are different entities looking into different possible crimes committed by the former President. When different entities can make a decision to bring a case or how they bring one is going to differ based on how long the investigation has happened, manpower, how much evidence needs to be examined, etc.
In this case, it just happens that that the Manhattan DA got there first. It is what it is