Why would someone have low cardiac output?

Congestive heart. Failure can be cause of low cardiac output. If the pump is weak and failing then cardiac out put would be low. Very slow heart rate can also result in low cardiac output.
Size related. Cardiac output is directly related to your body surface area (your size). Small people have low cardiac outputs. Perhaps you meant cardiac index? This the co indexed to body size. If it's low, it's because the stroke volume or the heart rate is too low. Sv can be low when the the heart is beating weakly or blood is moving in the wrong direction due to valve disease or a shunt between the chambers.