In addition to the reduced air density at altitude (as others have mentioned), body position, weight, and clothing also have a lot to do with terminal velocity.

In addition to the reduced air density at altitude (as others have mentioned), body position, weight, and clothing also have a lot to do with terminal velocity. A light person wearing baggy clothes in a belly-down position might fall at only 90 mph, whereas a heavy person wearing skin-tight clothes in a head-down position could fall at 170 mph. (speeds given at lower altitudes, close to the ground)