As far as I know, they use a CO2 meter because that's what is most important. When you're underwater for great lengths of time, I believe the buildup of carbon dioxide becomes toxic more quickly than a lack of oxygen. Think of the spacecraft Apollo 13 and the movie about it - CO2 buildup was the problem, not a lack of oxygen.
I know that the human brain detects/signals the need to breathe by sensing rising blood acidity - this is caused by increased carbon dioxide dissolving into ah carbonic acid I believe in the blood. So our brains detect CO2 levels in the blood rather than oxygen levels
