I went to the Arkansas Derby once. Despite the handicappers talking about mud, the track was covered with sand. It gets wet, but it drains fast and it's not sticky when wet.

In Missouri where I live, we get muds that are so sticky they could glue a horse in place. That is not what they put on the tracks.

Apparently some horses just don't like wet ground and others don't care. When the sand is wet, a bettor wants to know which is which.