To be atheist, as opposed to agnostic, is to find predictability and comfort in the belief that the prime constituent of the universe is matter. In this view, consciousness is an emergent phenomenon that arises from matter that has accidentally been organized sufficiently.
Belief in a materialist universe offers an illusion of metaphysical certainty. There's comfort in believing that our current scientific understanding is substantially complete, and substantially correct. That all that's left to work out are kinks and details. The belief suggests what is to be valued, and provides purpose: science, knowledge, technology, experimentation. The atheist points to the many successes of this process as evidence that brightness lies this way.
What the atheist gets from the materialist hypothesis is much the same thing that the Christian gets from God: answers of a firmness that otherwise could not be obtained; a metaphysical framework on which to build life, and find meaning. Much like Pascal rationalized the irrational using Pascal's Wager; or like Thomas Aquinas did the same with his five proofs; so the atheist does this using Occam's Razor - an excuse to believe that which has no proof. Like with a person who is religious, what compels the atheist to defend materialism so ardently is not ultimately reason; it is fear of losing the entire foundation of that which he's built.
The atheist fears that allowing investigation of a non-materialist hypothesis threatens to undermine science, and to impede the holy grail of technological progress, which the atheist believes is our main worthwhile goal. Anything doubting the materialist belief is seen as supportive of the religious; and anything religious is seen as a source of evil, backwardness, suffering, and darkness. Therefore, any suggestion of a non-materialist hypothesis is met with extreme prejudice, since it's perceived as threatening to the one thing which the atheist considers holy.
The atheist values investigation - but a non-materialist hypothesis is the one area he will not willingly investigate. You are either with us, or against us; and if you doubt the universe is made primarily of matter, you are against.