Here's the argument I've heard from show/race people.
Hay (no matter the quality) is unable to provide the shear amount of calories needed for these horses. They also have greater demands for other nutrients that hay is unable to provide. That is why they need hard feed.
Now I've wondered in the past if a high quality alfalfa fed in high quanities is capable of meeting these energy needs.
Granted most of the horses around the world are more than capable of meeting their needs on a good grass hay or pasture, but what about the extreme atheletes?
Extreme atheletes need more then hay can provide no arguement there many endurance horses are adding those calories with fat I don't think it will ever catch on for race horses too expensive :)