I think you can blame the original Zelda for the Japanese ramping up difficulty when they export games. That series is more popular in the Western world than in Japan, and the original probably defined Nintendo Hard.
However, that doesn't explain why Fire Emblem Radiant Dawn's (Path of Radiance?) Normal difficulty is the equivalent of Japan's hard. While the series is pretty popular in the West, I think it's still kind of niche.
It's more popular in Japan than in America, considering we never got the first six or so installments.
I just know we like Zelda more than they do, and the original two were brutal.
.... *Looks at his gold carts. Raises an eyebrow
Are we talking about the original NES Zeldas?