Why do some people get motion sickness, but not others?

Motion sickness. Motion sickness is due to "ocular - vestibular mismatch." the vestibular system is in the inner ear and senses position and rotation. When what the eyes see does not match what the vestibular system senses, it causes motion sickness. Some people are much more sensitive than others to this mismatch. You can reduce the issue by looking outside the boat or car. Then you have a better match.
Motion sickness. Motion sickness can happen if the signals from the brain from the eyes and from the balance sense organ (within the inner ear) and other cues are different. Differences in the function of the inner ear, vision, nervous system, or vascular system of the brain can all result in a tendency towards motion sickness to varying degrees. Everyone is different, so tendencies towards motion sickness vary.