As for the "why": I'd love to learn about all of the historical and regulatory incentives here. From what I've managed to cobble together, this is partly due to CAFE standards[1], partly due to Section 179 of the US tax code[2], and possibly(??) due to contractor demand[3]. [1]: In 2006, the NHTSA adopted a "footprint" approach to CAFE standards, where fuel efficiency standards are set based on the vehicle's wheelbas…
But I happily move sheet goods with a trailer now, towed by a light SUV. Even not having much truck experience, I'd say it's easier to load/unload than a truck (can you fork things from the side out of a standard pickup bed?). So if I wanted a truck for more towing capacity, the sheet goods thing wouldn't really matter as I could always use the trailer.
But I also think a lot of contractors tow enclosed trailers that keep all their tools and whatnot, so perhaps they're back to wanting larger trucks for times when they need one or two sheet goods easily (for larger quantities they'll often use delivery. I love those three wheeled rough terrain forklifts)
I'd think I'm solidly in the target market for something like a kei truck, but they don't have much appeal for me. Little towing capacity. Poor crash standards means they wouldn't be good for the case of driving a medium highway distance to pick up something off craigslist. Perhaps if one with a dump bed fell into my lap.