the scuba tanks are only 20% oxygen....at 5x to 20x the pressure of a space suit (the same volume of air has 20x more molecules, thus depletes the scuba tank faster - thats why scuba divers can stay at a depth of 30ft for many hours, but the same air supply may only last 10-15 minutes at 150ft. The scuba regulator must deliver denser, high pressure air to the diver, or his lungs can't overcome the 2x-20x sea level pressures.

Space suits use 100% oxygen (already 5x more oxygen than compressed air) they are also not pressurized to 1 atm, more like 0.4 atm, so they are also breathing gas at a much lower rate than a person on earth.