Because you're looking forward at the platform that's moving down and toward you. Kind of like if you were to stare down at the belt of a treadmill while it's moving - it would be disorienting after a while.

Doesn't this boil down to a claim that blind people are incapable of running on a constant upwards slope due to motion sickness?

Because even if your claim is correct, and the conflicting visual input is disorienting, the Moon-dweller on a similar contraception could just close their eyes.

Staring down is not equivalent to being blind. Motion sickness occurs when signals from your eyes (like the motion of the world around you), body (like wind on your face) and auricular semicircular canals (organs in your ear that sense acceleration) conflict with one another. In theory, your brain reacts to this with nausea because historically that situation would mainly arise due to illness or poisoning, so it might be good to vomit. That's why you can sometimes alleviate car sickness by opening a window (convincing your brain that you're definitely moving) or, if you're driving at constant speed, refraining from looking out the window (convincing your brain that you're stationary).

Blind people only have 2 out of the three signals so they might be less prone to motion sickness.