Originally Posted by Faulkner

You are right, cost is the issue which prevents the typical police cruiser from having at least some armor in the doors.

Now, if someone could invent a way where police cruiser door armor could be modular in such a way that it could be rather easily uninstalled when the car is removed from service (i.e., traded in) and reinstalled economically on the next car, that inventor would be our next millionare.