Another possibility for a reason for having a wooden bed in a p/u truck is that it made additions to the bed easier for the those who changed the bed configuration to better suit their needs. Farmers around here still add tool boxes, canopies to shelter field hands riding in the back, fuel tanks, etc, to their trucks.
Wood is more, "forgiving", when drilling holes. It also allows common fasteners like screws & nails to be used used by the end user to attach add-ons to a truck bed. It's a lot easier for most folks to work with than a steel bed.
Also, auto makers didn't have dies large enough to produce one piece steel bed floors until the mid or late 1930's. That's why we don't see one piece steel roofs on autos until then, I think.
I guess the same question could be asked about vinyl tops on non-convertibles still being used today. It continued making the appearance of an auto something a little, "sporty", compared to a regular hard-top.
