Putting it in two words: Wrong fire.
Actually, it comes back to my original reaction to Fahrenheit 9/11. When I saw the movie, I felt like asking "Is that all? What's the fuss about?" From the loud screaming and bitter noisy crying of the Bushevik children, I was really expecting a monumental epic.
Michael Moore made no secret that the movie was supposed to be a very negative attack on Dubya's qualifications for and handling of his "job". However, there was nothing extremely negative there. The background facts were old and well known to anyone who has been paying attention. A lot of the footage was just the end bits that regular media sources had snipped out of their broadcasts. Yes, Moore's perspective certainly didn't match the official propaganda very well, but the tone was basically light, even humorous. I certainly dislike Dubya and am receptive to criticism of BushCo, and I certainly would have been sensitized to notice any reasons to hate Dubya on some kind of personal level, but the movie didn't offer them. In conclusion, only a rather ignorant person could have a strong reaction to that movie--but the many strong reactions suggested that there were lots of such Americans...
Contrast it to the anti-Clinton rhetoric and the endless witch hunt. Maybe I was fooled, but they sure had me convinced that their hatred was real, very sincere, and very intense. Contrast it with the anti-Michael-Moore books and movies that were made in "response" to Fahrenheit 9/11. No way I could be mistaken about the palpable hatred there. Compare it with the attack ads from Nixon's revived swift boat vets and you can feel the purity and intensity of their rightwing hatred. Not just limited to politics, by the way. It's the same kind of irrational hatred that fanatically argues that 3,000 innocent American deaths (caused by Saudi terrorists with funding that originally came from BushCo) completely justify 100,000 innocent Iraqi deaths (even though Iraq had nothing to do with 9/11 and posed no significant threat).
In conclusion, Michael Moore obviously recognizes how negative American politics has become, and he wanted to fight that fire with fire, but he didn't have the right fire--the fire of wrongness. A bit of humor is no match for the pure, distilled hatred that drives so many of Dubya's supporters.