Question

Why was it difficult for American forces to defend themselves against Japanese fire

Answer 1

I don't know what you're referring to, and don't think it was much different than defending against a number of well-armed, trained, and disciplined troops. If anything, the German Army in WWII was a really tough bunch. 

Two things may show some differences, however, at least in the war in the Pacific. Japanese were far more willing to fight to the last man, rather than surrender, whereas most German units would stick the white flag up when it was clear that they had lost the engagement, and had no retreat. In "island hopping", the US also took on extremely fortified Japanese positions that were not destroyed with even massive bombing and naval fire