because water is a hard surface at landing speeds and the plane often breaks apart at contact. if you manage to land in one piece, you have limited time before plane starts submerging.
Hindsight is always 20/20. Without knowing how things will turn out, it seems reasonable that it would be safer to land on the runway without landing gear, even if water is available. Certainly in some cases it might turn out that a water landing would have been the better option, but you might not know that until it's too late.
