I haven't read John and Belle on this yet, but I suspect part of the explanation is parallel to the fantastic one s/he cited re Harry Potter: people like to argue about evolution because it's fun to be a part of the movement. It's a well-defined flashpoint.
In addition, there's the ever-irritating Moral Angle: evolution is more readily thought to have some relevance to how we do and should act, more so than, say, cosmology. Though notice that Biblical *literalists* should-- and in the case of the Flat Earthers, do-- have a beef with the physics and cosmology as well, time for reconciliation or no.
PS: this reminds me of another question I can't quite answer, viz., why is the pro/anti abortion debate so much on religious lines? That is, atheists (oh, sorry, "brights") are committed to "don't murder" as much as anyone else, and there's only the sketchiest of scriptural reasons for thinking fetuses are moral persons, or for some other reason within the scope of that principle, yet that's the way the groups always fall.
It is odd about abortion. In fact, you'd think the godless would be a lot more upset about taking away the one and only chance a being has at life. But probably the anti-abortionists would disagree that atheists are as committed to "don't murder" as much as anyone else. I would guess that the camps are composed the way they are because of the other issues surrounding abortion: liberty vs. the moral authority of the state, sex good vs. sex bad, etc.
I do think you're right about evolution as flashpoint. In fact, that's what I was trying to say in my last paragraph. But I'm not sure that the linking of evolution and ethics is a cause and not an effect. Plenty of conclusions could be drawn from differing views of cosmology (and this was a big part of the problem for the church in the time of Copernicus), but it seems it's just evolution's turn at the moment.
This marks the second time you've brought up Flat Earthers. Hmm.
John and Belle are separate people, which you may know. You can tell the author of each post by the accompanying picture. If you were unsure because you can't remember who wrote what, I apologize for condescending. If you didn't know about the pictures, well, duh.
Yes, their human offspring led me to believe that they are two, but I couldn't remember who wrote the Potter bit.
I don't think it matters so much that the anti-abortion theist seems to think that the atheist is immoral or amoral, not least because that's just silly. The puzzle is just where the two sides part company, and, given the rhetoric of the debate, it's not where you'd expect. Looking at a dramatically oversimplified version of the dialectic, both sides agree to "it's wrong to kill" (the overtly moral premise). Then they disagree about whether or not the fetus is a person-- a bit surprising, given that (a) this appears, I think misleadingly, to be a nonmoral claim, and (b) it's a claim that the Bible doesn't say so much about, directly. This suggests, as you point out, that the conversation doesn't reveal what's really at issue. (That is, if it's good sex/bad sex, this is utterly lost in the "sanctity of life/protect the innocent" rhetoric.)
Factoid: I have it on good authority that there's a very strong correlation between an American religious denomination's view on abortion and the roles it allows women in the church. Interesting if true, and more support for the hypothesis.