Well, I know two people quite well who oppose evolution very strongly, one twenty-something girl, and one sixty-something guy. The girl has an undergraduate biology degree, believe it or not.
The why in both cases is mostly religious. The girl with the biology degree was brought up in a very religious family, and more than that, she feels that she has to be
more religious than anyone else. She has security issues or something. So she adopts views and behaviours that will make her feel like she's more of a believer. She's also rather gullible (believes in homeopathy, naturopathy, fluoridation of water is an evil corporate conspiracy, et cetera). I'm not sure why, but if it has to do with health or biology, she'll latch onto anything that is ATM. We've had a number of discussions about evolution, and she seems to just accept ATM claims like "speciation can never be observed," and she just gets angry when I point out that it has been many times (and the next time the discussion comes up, she seems to have forgotten). Otherwise, a very smart person, though.
Same with the old guy: normally good logic, fun to spar with verbally, but just closes his head when things he doesn't want to hear are said. His method is funnier, though. Instead of getting angry, he just talks louder and louder, until whatever disagreeable thing you are saying is drowned out. Very amusing at restaurants.
My little sister, of all people, doesn't believe in evolution, either. However, she told me that she doesn't believe in it because she hasn't seen enough evidence for it yet. When I told her, "look, it's a fact," she replied with, "why should I believe you?" I wasn't sure whether to hug her or slap her.
I'm not worried. With that mindset, she's in good shape to go through life.