Why do they call Superman the Man of Tomorrow?

[The Frozen Reptile]

The epithet might have been influenced by the 1939 New York World's Fair, which had a "Futurama" exhibit that depicted "the World of Tomorrow.".

This is exactly what it is; at the time of the New York World's Fair in 1939, Superman was just starting to get known outside of the New York area and Harry Donenfeld had somebody hired to promote the event of Superman's appearance. What they came up with was the Man of Tomorrow inspired by the exhibit, specifically "The Man of Tomorrow at the World of Tomorrow". Of course, the World's Fair Comics were first sold here (purcesor to World's best/World's Finest).

[Super-Cyke]

Because Metropolis is called the City of Tomorrow. Superman made Metropolis his area of residence, and Metropolis' nickname brought a similar one to Superman