Hundreds of thousands of Hungarians and Romanians fought at Stalingrad, but most depictions of WW2 depict the conflict as Germany with some Italian help in the west, and Japan in the east. Why are the other three countries never mentioned?
The term "Axis" was first used in the mid 1930's, before the outbreak of shooting in World War II, when Nazi Germany and Fascist Italy made a diplomatic alliance, and Mussolini declared that "Rome-Berlin" would be an "Axis" around which the rest of the European continent would revolve, the implication being that Rome and Berlin would be the center of power in the coming political order of Europe.
So, the term in its most original usage applied only to Nazi Germany and Fascist Italy, a boast by Mussolini.
Later in 1940, Germany, Italy and Japan made a formal treaty of nonaggression and mutual alliance called the "Tripartite Pact". It essentially pledged the signatories to support each other's ambitions and to not make alliance with each other's enemies and rivals. Other countries would later sign this treaty and formally "join the Axis powers", although Finland never did, they did fight against the Soviet Union.
It is definitely true that the Axis powers had allies in Europe such as Hungary and Romania, and they contributed troops that fought with the Germans and Italians in various campaigns during the war. But the "Axis" most popularly refers to Germany and Italy, with Imperial Japan coming to be included under the label later in the war, even though the other countries you mention do formally join the "Axis" later and it's not incorrect to call them "Axis Powers".
To answer your question directly, then, it's a matter of historical timing of the term, that at first the "Axis" was only Germany and Italy, and perhaps also the sense that Germany and Japan were the major part of the military strength of the Axis. I don't think it's true that they are "never mentioned", they're just not mentioned as often due to their later, smaller roles in the conflict.
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