I have a hard time with expanding this generalization to the entire US. Here in the Boston area, 1960s and 70s three-speeds continue to be plentiful, and that's likely because Raleigh USA was headquartered in Boston for decades, and the distribution channel/supply chain was in place. If you didn't see three speeds in your shop in the middle of the US, it's likely because the supply chain never made it out that way.