And the athlete sponsor has to be able to justify the investment by tying the investment to the additional sales it generates.
Just as with basketball, football, tennis, etc. . . . the athlete's value is tied directly to their attractiveness. (There, I said it.) Most athletes that pull down the big endorsements are little more than models and/or spokesmodels. A championship title of some sort CAN help, but isn't really necessary. If they look GREAT in a swimsuit, their sponsorship value as an "athlete" is multiplied. If they are physically attractive and articulate and the sponsor wants TV commercials, the value as an "athlete" is multiplied.
I guess what I'm saying is, an athlete's endorsement value has a lot less to do with their athletic achievements than it does with their physical attractiveness -- both male and female. Less than 1/100th of 1% of the target audience knows or cares who won Kona this year. (Heck, most of the target audience doesn't know what a Kona is!) Find a stunning male/female model, enter them in a triathlon if you must, and pose them with a bike or a set of goggles in a tiny swimsuit. There's your value proposition. Eyeballs always win.