Bah, it's a shitty play on the fact your peripheral vision is appalling.

If you want a better party trick, draw a cross on the paper and a dot about an inch to the right of it. Close your left eye and stare at the cross and only that. While keeping the dot off to the right side, move the paper forwards and backwards slowly. Eventually the dot will enter the portion of your vision where the optical nerve bundle exists the eye and will seem to "disappear".

Watch it a few times, once looking at the faces, and one staring at the cross.

I'm going to say it's because their brains are boring and don't know how to make pretty women into ogres like the rest of us.

It's...there's no illusion to it, just hard to process fast flashing images that you're supposed to focus on from the weak point of your eyes. Also a good number of those ladies would not be described as "classically pretty" to start with.