For one thing, they don't want Humans to be "descended from monkeys" (as they like to put it). For another thing, teaching evolution (and science in general) in schools is seen as a threat to their historic use of the school system for indoctrination.
You know, for non-Americans, this is so strange. I went to school in the UK, in Brazil, in Germany and in South Africa, and never once was I even aware of "creationism" before I went to the USA. No-one in Europe, South Africa or Brazil would ever think of contesting evolution (as far as I can tell, of course), because, as Sheldon Cooper puts it, it's not an opinion, it's a fact.
It irks me how "Christians" MUST be bigoted people who believe in this creationism stuff, are against gay rights or a woman's right to terminate a pregnancy - this is the vibe you get on Facebook, anyhow, from virulent atheists. That believers are stupid people who believe in an Old Testament God.
On the other hand, a lot of religious people calling themselves Christians rely too heavily on the Old Testament, when really, that's just sort of a historic chronicle, and not a means of justifying hatred against those who are different from you. The Christian bit of the Bible is actually that one pertaining to Christ. Just saying.
I'm a Catholic, I believe in God, I believe in evolution (because it's a FACT), I support gay rights, emancipation, peace and love and tolerance...you know, that stuff Jesus Christ preached, back in the day. When has Jesus ever
said anything about discriminating gay people, prohibiting abortions, forcing your theory of how life came to be down people's throats? He didn't. What he did say was that before finding flaw with the others, do yourself a favour and find yours, first. Leave each other alone. Be nice. Be TOLERANT. Treat others the way you want to be treated, which includes their belief system.
Neither evolution, nor opinions contrary to mine threaten me, my way of life, or anything / anyone else. Claiming that is just plain idiotic. And utterly baffling.