All joking aside, an interesting comment was made a few weeks back and I can't remember who said or where ... It might have been Stovebolt HQ neighbor and past ATHS National President, John V at the Leonardtown Show ... ??))
But that the 9-ft bed was offered because most saw mills at the time only produced rough cut lumber to true dimensions. And 8 ft was a standard length. But it wasn't *exactly* 8 ft. It might vary a few inches (but never *less* than 8 ft). Thus, most of that standard 8-foot lumber would not fit in an 8-ft bed with the tailgate closed.
I have no idea as to the veracity of this. But it *is* an interesting thought.