Iron was revolutionary because to make Bronze, you needed to mix tin and copper, and neither seemed to have deposits near each other, making trade, or war and occupation necessary to obtain a steady supply. Iirc, bronze swords can’t exist past a certain length, or they become fragile.
Iron, on the other hand, had a much higher melting point, making it hard to melt and harder to purify iirc. Still, you only needed just the iron. You could make iron alloys or steel or whatnot to make something better but that’s not the point.
Edit: All of Japan’s iron deposits were mostly low quality. Additionally, they didn’t have the technology to properly melt the iron and purify it. This is why they had to come up with the risky “folded 1000 times” method, that can potentially beat too much carbon (or something) out of the metal and end up making the end result brittle anyway.