To put it bluntly, racism.

1940's America was still very much a racist country. Foreign minorities had been persecuted and campaigned against for decades. The military was still segregated and Jim Crow laws were still being felt by many Black-Americans

In the White-American's eyes, the Japanese were beneath humans. Often portrayed as monkeys or snakes. The Germans however, were human. They were after all descendants of Europeans, just like most Americans. Because of this ancestral/racial connection, many thought that there were "good Germans", but such a thing would not be said of the Japanese.

There was a popular slogan during the war (I believe it was in the NY Times) that read something like, "There will be no more Adolf Hitler and no more Yellow Japs." If we examine this slogan we find something odd: America's enemies are Hitler and the Yellow Japanese. Notice that it is not the Germans that the Americans are fighting, but Hitler. They wanted Hitler dead, not the German people. The Japanese, well they could not have cared less about them. It clearly says that they do not want anymore "yellow japs". I'm sure you know that in the modern day, that calling a Japanese person (or any Asian for that matter) yellow is entirely unacceptable. However, in a society where the Japanese were beneath the white European, few would bat an eye at such a remark. The Japanese were truly seen as non-human beings. Some American GIs were even given a souvenir "Jap Hunting License."

It is interesting to note that German-Americans were not sent to internment camps like the 110,000 Japanese-Americans.

The superhuman imagery comes more from the battlefield stereotypes. The Japanese soldiers disregard for life played heavily Into the superhuman aspect of propaganda. The average soldier would likely surrender when overwhelmed, but the Japanese looked down upon those that surrendered, often fighting until their death. This gave the illusion that the Japanese could not/ would not be defeated. Every time one was killed, two more would take his place. The Japanese were also portrayed this way in the hopes of frightening the readers. Often times, when a Japanese soldier is portrayed as super human, it is related to a lost battle or some sort of war atrocity (I.e. Bataan, Nanjing, etc.). The use of superhuman characters gives a reason for the allied defeat. These cartoons are essentially saying, "The Japanese did not win because they are better than us! It's because they are super beings!"

My primary source for this is "War Without Mercy" by John Dower, which is great for this topic.

Hope I was thorough enough for you!