" Why does that even happen? Is there any science behind why some people have the problem while others don't? "
The only thing I can conclude is that it's simulating movement, and I guess visually, a simulated, floating projected image can twist some people's stomachs.
I don't know if this is actually true, but in middle school I remember my science teacher saying something about motion sickness being the body's response to a disconnect between eye and inner-ear signals. When you're reading in a car, your inner ear senses that you're moving, but your eyes are focused on the stationary inside of the vehicle. When you're playing a game, your inner ear senses that you're sitting still, but your eyes are telling your brain that you're running around in a virtual environment.
The reason people feel nauseous is (supposedly) because the brain says "whoa whoa whoa hold the phone! One sense says I'm moving and one sense says I'm NOT moving. They can't both be right, so SOMETHING is messed up with one of them. I don't know why one of them is messed up, but I'm going to assume it's some kind of poison that my stupid body ate. That poison needs to go, so I'll send some pukey signals to this idiot's tummy to flush that garbage out."
Again, this may not be the actual mechanism or cause of motion sickness, but it seems like a reasonable explanation to me.