Ethan, let me start by saying I agree with you, I find this all fascinating. I have done a handful of research into why this is, and here's what someone on the other side of the issue might respond:
Catholics are not "Real Christians". In fact, the Catholic church is an abomination, and possibly the anti-christ. One cannot reconcile evolution with Genesis 1:1, when taken literally. Catholicism is much more appeasing to the idea of taking certain parts of the Bible figuratively or symbolically. Many of these other "fringe groups" you speak of take the Bible VERY literally, and will not accept any flaws.
The whole idea of the anti-evolution movement, besides the desire to end up with a theocratic society, is really to be rid of any insecurities about a "true knowledge" of the supernatural. If something challenges it, it is automatically untrue - the scriptures win out over any contradiction. There is no compromise.
Another way it was explained is this:
1. Evolution claims death existed before mankind. (Plants died, dinosaurs died, early mammals died, etc)
2. According to the Bible, death didn't exist until "the fall", which was *after* the arrival of mankind - a penalty for misbehaving
3. If death was not a punishment (as evolution would contradict), then Jesus' death, meant to save mankind from death itself, was misplaced - and the whole idea of the death and resurrection means nothing.
Therefore the claims as given in the Bible are misplaced and/or false - and since the Bible claims to be the word of God and a "perfect book", it is wrong, and therefore the whole of the Bible is wrong, misplacing the actions and beliefs of billions of people over the last 1700+ years.
Granted that some of these logical claims are a bit extreme, but you see where they're getting this from, and why evolution is so basically offensive to the creation story - and to some specific religions.