Yeah, I'd agree that it's a combo of cost, availability, and reputation. Zipp and Michelin tend to be higher priced than Conti, especially for the "good enough to race and train" crowd. If you are an LBS shopper, picking up tires and tubes added on to other bigger purchases, Specialized and Bontrager (in particular) are clearly biased to certain shops (typically non-tri-specific shops to boot). Schwalbe just isn't as prevalent. And Vittoria definitely suffers in reputation versus Conti, although (therefore?) you can always find them inexpensively somewhere. ETA: ...for clinchers. :)