I've got a better question.
Hundreds of road marathon specialists finish behind the best ultra runner in the world. They can't finish in the top 20 of the US trails, probably not top 1000 in the world at marathon..... why aren't they doing something else.... like running ultras...?
Its clear that these runners will never be fast enough to make the Olympic Marathon team, or earn a living on sponsorship as a road runner.... but in the still developing and growing world of trail and ultra running they still have a chance for a career and for growth.
And I answer:
1) Many young people still don't even know that trail and ultra exist.
2) But many do, and an growing number actually are choosing trail/ultra over the mainstream system in recent years, rather than giving up running altogether.
3) There is a weird disconnect between types of people. Some are drawn to the mainstream, popular, financially lucrative thing. Others are drawn to the counter culture.