Moore is not interested in exploring complexity. He likes to find scapegoats - generally some mix of corporations, America, and "stupid white males" - and hammer them. To properly arouse anger at his scapegoats, he ignores nuance and subtlety, and misrepresents statistics. It is not enough to criticize the NRA - Moore needs to make Charlton Heston look like a racist. It is not enough to find out why other countries have a lower murder rate than the US - Moore needs to make the difference in murder rates sound 20 times as bad as it is. And God knows he can't admit that in other crime categories America is actually safer than other countries. Then people might actually take a balanced look at the US, instead of finding America despicable.
Moore is an extremely funny commentator, and he's a talented filmmaker. Often his observations about politics are bluntly true, and need saying. But he wastes his talent on hatchet jobs. An honest and thought-provoking documentary about violence in America would be extremely valuable. That Michael Moore is incapable of making such a movie is everyone's loss.
That Moore is the number-one lefty commentator of the moment is explanation enough for why the left has lost the moral high ground.