I could be wrong, but here is my thought process on this.
As you get deeper you would need to ensure a tighter and tighter seal (from not really needing any seal near the surface) to be able to breath in any of the air. I base this on a simple experiment. Take a hose and try to breathe through it when you are 5 or so feet under the surface and it isn't too hard. Try and breathe through it at 30 feet and you are gonna struggle more. As you go deeper your lungs (an airspace) are compressed and therefore you need more air pressure to overcome the resistance to breathing created by the outside water pressure. This is what your reg does automatically for you.
Doesn't this work the same for trying to breathe from a tank? Wouldn't you need a better seal as you go deeper so that you can build that pressure to a point that it will overcome the outside pressure.