A reader proffers an answer to the question:
Because, speaking as a former Baptist, there is a correlation between being a Baptist and saying and believing things that are blush-inducing to non-Baptists. Off the top of my head, these include making straight-faced professions of belief in the imminence of the Rapture (where the faithful will be magicked up to Heaven while the heathen masses endure a Wagnerian spectacle of Hell on Earth) and maintaining that the Grand Canyon was formed by the 40 days and 40 nights of rain that gave everybody but Noah so much trouble, and that the ubiquitous and overlapping evidence for an Old Earth is a Baroque kind of divine fraud.
We ridicule people who believe absurd things for non-religious reasons all the time, and nobody bats an eye. Think about how the media represent believers in alien abductions -- if you believe that aliens in UFOs are currently invisibly abducting thousands of Americans, you're branded a lunatic. If you believe that God is planning to invisibly abduct millions of Americans a few years from now (and that you will be among the elect), you have a good shot at a political career in the South.
I wouldn't be quite so blunt. But for me, the evolution issue is very hard to get past. Those who believe that the earth was created 6,000 years ago and that human life has not evolved from more primitive forms are people I cannot engage with in civil discourse. To posit faith in things unprovable and unknowable is one thing. To posit faith in something demonstrably falsifiable is another. I simply have no tolerance for creationism or for those who enable it. Creationists are as much an insult to reasonable Christians as they are to rational thought. And they perpetuate the notion that religious faith is indistinguishable from idiocy.