A couple of reasons:
1. Hard to deal with steering--kick up both ways etc.
2. Tacking up narrow channels--you'd have to shunt, bear off and get on the new tack in the narow confines.
3. Not too many folks have really pushed the technology gains of other multis in this direction. Efficiency gains of the Tornado have been bankrolled by Olympic campaigns. Who would bankroll this effort?

That said, an offshore course might be an ideal spot to test the potential efficiency. Proas are best when long and slim for super narrow L/B ratios. I suppose one could get a proa of 30 feet length to weigh what a 20' cat might weigh.

Randy's entry really piques my interest in that he beat a pretty well sailed Tornado into the first checkpoint by 24 minutes. It's obviously fast--but so is Randy. It kind of reminds me of a double outrigger sailing canoe rather than the modern tri. No 2-300% amas, instead body weight hiked out like an International canoe on a rack--but with a little ama undeneath to eliminate those occasional bobbles which would make an IC sailor swim.