Why cant a scuba filled with oxygen only?

Scuba tanks get hot when you fill them. Putting them underwater keeps them cool. If they overheat, the burst disk may blow. But even if it doesn't, a "hot" fill will cool down to a lower pressure, so if you think you have filled the tank to 3,000 PSI, when it cools down you will find that you only have, say, 2,750 PSI in your tank.