The people in this country for the most part have a level of allegiance to their government which far exceeds that of the little militias in Baghdad. Such allegiance would be stronger if people felt that the government viewed them as a partner and ally in maintaining the peace, rather than as an adversary.
Some do, and hopefully it's many, but right here on FR I have seen many posters who, because they don't like certain USSC decisions claim it to be time to take matters into their own hands. And then, of course, the other groups I mentioned with no allegiance to either the US or the Constitution would continually provide constant threats to the local police. And if you tell me that no problem, the militias will come in and help out, what do we then have? The group with the most toys wins. And it would merely provoke thousands of mini-arms races across the Nation. Does that sound like what our Founding Fathers had in mind?