You're listening way too close, burgermeister!!
I think it has to do with the size and quantity of the bubbles. In order to provide fish with oxygen, the bubbles have to be extremely numerous because the cumulative surface area is so much greater for the same volume of air. I think that is one of the reasons the system is so expensive because it requires large volumes of air in the form of extemely tiny bubbles. The bubbles essentially stay entrained in the hypolimnion because each individual buble is so small. You don't get the added benefit of air/water interface from traditional airlift diffusion.
P.S. That might be the biggest B.S. answer ever given on the forum. :p
