The air compressor used to fill a tank to 3000 psi either works or it doesn't. There is little to go wrong. Impurities can enter compressed air but that problem is preventable by good maintenance and proper positioning of the compressor with respect to environmental exhausts. Nitrox is different. A couple of percentage points one way or the other could make a difference in the safety of the dive.
Nitrox experts recommend that the composition of tank gas be measured before each dive .
A second reason nitrox is not routinely used is that it requires additional training. Air-trained divers are not trained to dive with nitrox. Nitrox diving requires more education about the risks of oxygen toxicity and the importance of measuring the gas composition before diving. The third reason has to do with differences in philosophy. People of this philosophy readily concede the potential benefits of nitrox, but believe the potential hazards far outweigh those benefits for recreational divers and the recreational diving industry.