Because when you sit in the front seat, ob<x>ject are coming towards you and you can look at a fixed point. In the back seat, ob<x>jects are moving past you at a high rate of speed and you can't focus on a fixed point.
I think the rear of an auto tends to "swing" more. If that make sense. The ride in the back tends to be a little rougher. Plus watching the things outside passing by...as opposed to looking straight ahead at the road or as you are approaching something, can make you queasier faster, I would think. <br />
Just speculating though.