Track is a sport where there are differences between the physiology of 400 meter runners, 800 meter runners, 1500 meter runners and 5k runners. It specifically articulate these differences by provides multiple events that capture niche skills. Think of UTMB as a completely different classification. There is no guarantee that any elite marathoner would dominate UTMB. The climbs are incredibly steep and there is simply a different physiologically skillset required. Can any elite marathoner carry the loads of Sherpas in Nepal? Many Sherpa are not any bigger than elite marathoners. Its easy to continue with the power hiker analogy, and it is kind of funny, but these UTMB winners are very elite in their own sport and the UTMB races is arguably the biggest race in the world. Viewership is extremely high and it has captured the public's imagination in France etc. What race has done that in the USA? Would you rather watch beautiful scenery in the Alps or suburban go-getters running mid times in Boston? Trail running is a niche sport in the U.S. but enormous and mainstream in Europe.