Cotton's non-wicking properties are part of it. The other part of it is the only reason to be wearing clothing while hiking is for insulation. Dry cotton works well for this purpose. Wet cotton is no longer insulative. Wet synthetics do not lose their insulative properties to nearly the same degree, and they dry MUCH faster.
If you're hiking in jeans, a tee shirt, and a cotton sweatshirt in 40 degree weather and get hit with a freak thunderstorm, you are in serious trouble. If you're hiking in all synthetic clothing, just keep moving. You might get uncomfortable, but you won't get hypothermia.
Polypropylene is good, capilene long underwear makes a great thermal layer, synthetic fleece is okay, nylon is fine. Even wool has its uses. I knew one guy who hikes in 100% polyester work shirts. Don't carry cotton.