Too Many Sponsored
Seems like there are actually too many sponsored athletes. These brands are wise to hold back and sponsor very selectively. Track & field is such a niche sport that few people follow. And most sponsored track athletes have little-to-no impact on shoes/apparel sales.
At the same meet where 43% of top 3 finishers were unsponsored, there was a whopping ~3,000 fans in the stands each day. Not a good return on investment for the brands, especially if this is supposed to be the marquee event of domestic track & field marketing!