I dont think this is the case, its not the US college coaches detering these kids, consider on the womens side BYU, Arksasas, Colorado, Villinova, Stanford and Oregon all have signed or current distance runners competing in the meet, thats many of the top teams in the NCAA. NC State didnt have a competetive eligible runner this year, but did send Savanah Shaw to worlds a few years back.
Also, Oklahoma St sent one current athlete to try to make the team and one OSU unsigned recruit is on the team. Natalie Cook scratched Brooks PR so that could explain her abscence.
I dont think their are many USA U20 runners that would have turned down a spot if they were just given out like other countries do and didnt have to be earned. The team kit alone is worth thousands of $ value plus the prestige and free travel. The U20 is tough meet with big expectations for many. After a long season many athletes dont want to stretch their seasons.
Another key issue is USATF drug tests everyone who makes the team - this is a deterent for why some dont try to make team.