I can't explain why pro racers aren't choosing 'bents to race and set speed records. I can only guess that it's tied up in marketing, racing rules and sponsorship money. But as far as recreational fast riders on road bikes, I would still contend that they avoid 'bents because they are following a fashion. Even "uprightbent" says that most racer boys "wouldn't be caught dead" on one. Yup, that's what you would say about a country club wife wearing last year's dress.
It's no secret - it's old news. The aerodynamic riding position of a recumbent offers a significant speed advantage for most fit riders. Of course there are lots of other reasons to enjoy riding uprights.
Hey, Laurel Lane, this is funny, but true. I am about to step out the door to head to a ride in Essex County, MA with a couple of other "old guys" on 'bents! (Haverhill to Plum Island and back) We will ride about 50 miles, and average 19-20 mph over that ride. Keep on the lookout!