Because they can see a disproven scientific theory for what it is.
I suppose an opinion poll is too crude an instrument to find out what kind of evolution various numbers of people believe in. It would seem to me that there’s probably three rather than two groups of opinion, one being an atheist “process of evolution” school, one being a form of intelligent design that incorporates some evolution, and a third school that believes all things we see were created more or less simultaneously, then that divides into a six thousand year camp and a less literalist camp. So actually four schools of thought. Mark me down as “don’t know” because that’s the only really defensible position to take in all of that.