Because if you, as a conservative attempt to point out logical fallacies, unsubstantiated claims, erroneous assumptions or false premises, they will not attempt to counter your argument. Instead, they'll call you a name and accuse you of something.
They do seem to like to present their “arguments” in forums where they will not be challenged.
I assert that this is why they were so mad at Juan Williams about serving up the low hanging curveball for Newt to hit out of the park.
It’s not that he was making a “softer” argument than any of them ever do, it’s just that he did so in a forum where the assertion could be challenged.
I think it is because the left argues in emotional terms, not necessarily rational. When cornered, they retreat to what they know. Emotion.