Why? Because it isn't true.
Fundamentalist Christians are my brothers and sisters in Christ, and I owe them support for that if for no other reason. But beyond that, it's simply not true that they're especially evil, other than in the scriptural sense that we're all evil due to original sin. Since about the mid '80s I've come to know more and more fundamentalist Christians as friends; as individuals. They don't tend to believe in evolution and they do tend to have right-wing politics, but they're not Nazis. For example, one fundamentalist friend of mine thinks homosexual sex is intrinsically sinful, a view I don't share. But he would never try to throw people in jail over it. He has no desire to impose his morals on the rest of society by coercive means. And I've found that a lot of fundamentalists are like that -- they are very far right on social issues, but they would never use the coercive power of the state to enforce their views.
Evolution? Most fundamentalists don't believe in it and think it's horribly wrong, but does that mean they're stupid people, blindly fighting science because it conflicts with their religious views? Sure, in some cases. And in others it's completely untrue. At some times in history the anti-evolution forces were -- in political if not in scientific terms -- the good guys.
Did you know that evolution was used in this country as the justification for laws that sterilized thousands of people without their knowledge and consent? Such laws existed in Virginia, for example. On "eugenic" grounds, people classed as "morons" or "idiots" -- very often on the basis of flawed tests developed by people working from theories now known to be wrong -- were hospitalized to treat things like appendicitis, and sterilized while they were under. That was a real evil. The pro-evolution folks in the '20s were all for it.
Now of course evolutionary biology isn't responsible for misuses of it by political crackpots. But the distinction isn't always easy to draw. When governments were officially religious, atrocities were often justified in the name of religion. Now that governments are more likely to base policy on "science," atrocities are justified in the name of science. Some (not all) people promoting evolution may be pushing an inhuman agenda even if their agenda doesn't logically follow from their science. Think of Richard Dawkins, who wants to close down religious schools and treat religion as a "mental illness," or his mentor William Hamilton, who wanted a worldwide program of eugenics using abortion, sterilization and infanticide. Creationists may not understand biology, but they are often quite perceptive about who the bad guys are.
Does this mean I don't defend evolutionary biology with these people, or liberal/libertarian politics? No way. On the former, I spend a lot of time on AOL message boards, arguing with creationists that evolutionary biology is not an attack on their faith. But I keep running up against the same fact -- most of the people on those boards who claim to be there to defend evolution are actually there to use evolution to push their brand of atheism. They are mostly Christian-haters, and sadly, most of them don't really know much about evolutionary biology. Some of the statements they make about "science" are every bit as stupid and laughable as anything the creationists say. The sad fact is that -- with exceptions on both sides -- in general, in my experience, the creationists are nicer people.
Don't denounce "fundamentalists" if you've never met any. My best friend in the world (other than my wife) is a man who once, after getting home from a long day at work himself, drove a dozen miles to my workplace to give me a jump for my car when I'd left the batteries on. That man is a fundamentalist. Another man, who lent me desperately needed help, financial and otherwise, during a long period when I was unemployed, is -- you guessed it -- a fundamentalist. The same man has been known to defend complete strangers from attack by criminals on the street, despite putting himself in danger to do so. And he would never think of asking the people he was defending whether they agreed with his views on religion or not.
I know atheists and believers in "liberal Christianity" who are also nice people. But a lot of them think it isn't a sign of religious prejudice to denounce "fundamentalists" and to stereotype them as a group. I used to think that way too. By God's grace I'm learning not to.