The Huffington Post has an article which points to a couple of University Egg Heads writing a paper bemoaning the fact female athletes get less endorsement deals for a few reasons.
The first being sexualization. It doesn't appeal to the target market. The second, according to the article, is the fact that a lot of the female athletes being used are Olympians, and therefore only "relevant" every four years or so. The article points to the Got Milk ad featuring Olympic Medalist Dana Torres in a bikini. The researchers write:
"Featuring Dara Torres as a middle-aged single mother, able to balance family with work commitments, might be more effective than highlighting her physical attractiveness at age 40."
Yes. Because apparently the calcium in milk is very effective at helping middle-aged single moms juggle their work-life balance. As opposed to building strong bones. Which allows you to swim better. Since Dan Torres is a SWIMMER. Last time I checked, you generally adorn yourself in swim wear whilst swimming.