The wood bed floor was a step in the evolution of the truck. Originally, the entire bed was wood, and was usually made by the owner. As technology improved, the bed sides were made of steel, but wood was kept for the floor.
It seems reasonable that the wooden floor was cheaper than steel. And until manufacturing improved, allowing the strength of a ribbed floor, wood was far lighter, which allowed more payload. Ad to that the "we've always done it that way" attitude, and you might have the answer.
