Well, basically because without NAD plus molecules, glycolysis will not take place. So things are pretty simple during the presence of oxygen, when we have enough oxygen present in the cell. But what happens if we do not have any oxygen present in the cell? What happens if we do not have enough oxygen present in the cell? So these types of conditions when we do not have oxygen are known as anaerobic conditions. And under anaerobic conditions, glycolysis is the only method by which we actually produce ATP.