There are a few factors at work here. Tojo didn't have anywhere close to the cult of personality that Adolf Hitler had. The only Japanese leader who's cult of personality could come close to rivaling Hitler, was Hirohito's. Therefore there is definitely a recognition factor at play. Hitler would have been instantly recognizable to anyone, where as Tojo was relatively obscure and out of the limelight until his appointment as Prime Minister in 1940. Now this isn't to say people wouldn't have recognized Tojo, but he wasn't as "famous" as Hitler, this isn't the main factor (I'll discuss that next paragraph) but it probably played a small role in the making of the poster.

There is also a good amount of racism in play here and its arguably a bigger factor. American propaganda sought to dehumanize the Japanese in a way that wasn't applied to the Germans. John Dower writes:

While Hitler and the Nazis also occasionally emerged as simians, this was a passing metaphor, a sign of aberration and atavism, and did not carry the explicit racial connotations of the Japanese ape

Where as Germans were still "people" the Japanese were subhuman apes who were all the same. Hitler was more out in the open and easy to target where as the Japanese were just a monolithic entity. Propaganda from the era tried to stress the point that all the Japanese were backstabbing, apes who were the bane of western civilization. Again to quote John Dower:

The Times’s practice of featuring political cartoons from other newspapers in its Sunday edition provides a convenient source through which to gain an impression of the thoroughly conventional nature of the simian 􀉹xation. Thus, in mid- 1942, Japanese soldiers in the Aleutians (who died almost to the last man) were depicted as an ape on a springboard. Later that year, the Japanese took the form of a monstrous King Kong 􀉹gure with a bloody knife. In 1943, they were portrayed squatting in the trees among monkeys, and on a wanted poster in a cartoon entitled “It All Depends on the Neighborhood.” In the latter, Hitler was “Public Enemy No. 1” for England and a jug-eared, monkey-faced Japanese was Hitler’s counterpart for the Allies in the Pacific.

As for the perception of Tojo by the allies. He was thought of as the "typical Jap", he was think with a long lanky torso, he had a flat pug like nose, a rather large head and very yellowish skin. Basically all these features became the stereotypical Japanese person that was featured in the media and allied propaganda. Everyone used this depiction of the Japanese, the Dutch, the British, and especially the Americans all took their depictions of Japanese people from Tojo.

The best source for this is John Dower's book "War Without Mercy".

Here is a picture put in the Times, about how to tell a Japanese person from a Chinese person; it showcases a lot of racial stereotypes for both peoples. http://imgur.com/ilQsiKc