Glad you asked. Because pros are your sport's representatives/advertisements while age groupers are your customers. Ironman loses money on the pros as a loss-leader (only $800 for all the triathlons they want and also paid prize money) for advertising to attract age groupers ($600 per race and no prize money). As reps of the company, you configure the pros they way you want the customers to be. This is why all sports do equal numbers at the top - it drives signups at the bottom.

Let's say you're a computer company and you want more African Americans to buy your products. Obvious step one is you put more African Americans in your ads. The pros are part the ad campaign for Ironman. If you want more women to sign up for your events, you put more women in your ad than there are currently in your (lacking) customer profile. It's marketing 101.

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Zen and the Art of Triathlon. Strava Workout Log
Interviews with Chris McCormack, Helle Frederikson, Angela Naeth, and many more.
http://www.zentriathlon.com