Why would standing still for long periods of time decrease cardiac output?

Answer 1

Cardiac Output is equal to the amount of blood each beat of the heart pumps times the rate the heart beats.

Therefore, to reduce the cardiac output, you either have to reduce your heart rate or reduce your stroke volume.

When you are running and exercising, your heart beat goes up (to get more oxygen and nutrients to your muscles, etc), thus your cardiac output goes up.

When standing still, your heart beat goes down, thus your cardiac output goes down...

More info than you probably want is here:
http://en.wikipedia.org/wiki/Cardiac_output