Because tribal affiliation is completely unassailable by logic.

We are all still tribal, barely evolved monkeys, and our manifold cerebral cortex is merely a tool to invent rationalizations for the tribal emotions that well up from our limbic system.

This is, of course, on both sides.

On HN, the effect is even more pronounced, because "I am smart, therefore my opinions are more valid than average" coincides with "I have concocted this justification for my hypocrisy" to grant even greater surety in personal belief.

The LessWrong folks worked pretty hard to inform people of these sort of biases, but the effort thus far has completely failed.