Why does a lack of available oxygen stop the aerobic metabolism of glucose?

At the end of the process of aerobic metabolism electrons must be removed from the electron transport chain. The electrons are reunited with their hydrogen ions and combined with oxygen to form water. If sufficient oxygen is not available a traffic jam will build up back through the system. Initially the traffic jam will be in the electron transport chain and the effect will be to prevent NADH and FADH2 delivering the protons they are carrying to the electron transport chain. Because they can't offload the protons they are not available to collect protons from the Krebs cycle and so the Krebs cycle won't be able to function. The traffic jam will work its way back through the system until there is an increase in the amount of pyruvate in the cell. The pyruvate will be converted to lactate to enable glycolysis to continue.