Gunther, on the earlier cars, up into the 1930's, the doors and most of the body on the "Woodys" were made of wood.
Everything (mostly) from the windshield back was mad of wood.
As the years went buy, more and more steel was used until it got to the point where the wood was no longer a structural member, it was only there for looks.
I think the reason the Station wagons started being made out of wood was because the automakers didn't catalog that type of body in the beginning and they were made by outside coachbuilders.
And, the technology to press the large sections of metal that would be needed to make the low sales volume SW bodies was not available and or too expensive.