Maybe because the art and the text are flowing in opposite directions? It's hard on the eyes, and I think it really takes a long time to adapt to that--not reading from right to left, but reading in two directions at once.
Reading unflipped manga used to give me splitting headaches. It doesn't anymore, not after years of practice, but it was a big part of my irritation with the move from flipped to unflipped, particularly with the trumpeting of unflipped as more "natural"--it was not the intention of the author that I suffer painful eyestrain when I read their comic, dammit.