Because it is: carbon dioxide does not have a chemical reaction to make the body oxygen-deficient, carbon monoxide is and human blood has a reaction to make people oxygen-deficient.

The mechanism of carbon monoxide poisoning is: carbon monoxide and hemoglobin affinity than oxygen and hemoglobin affinity 200-300 times higher, so carbon monoxide is very easy to combine with hemoglobin, the formation of carbon-oxygen hemoglobin, so that hemoglobin loss of oxygen-carrying capacity and role, resulting in tissue asphyxia.