Some languages are just more fun and more productive to work with. There is no Golder Hammer, but there are jackhammers, and I'd sure as hell rather break through a rock with one of those than a hammer.
Actually, there have been many Haskell programs winning programming contests. A lot of times, the programs are like 3X smaller and get finished long before non-Haskell competitors.
although "functional programming" is in the name, any language can enter. lots of Haskell and other FP winners in there. Although in '08 a Java team won..
