Why did John Shea leave the cast of and Clark The New Adventures after Season 1?

John Shea, who played the role of Clark Kent in the TV series "The New Adventures of Superman," did not leave the show after the first season. He was the lead actor for all four seasons, which aired from 1984 to 1987.

It seems you might be confusing him with another actor. Perhaps you are thinking about Dean Cain, who played Superman in the TV series "Lois & Clark: The New Adventures of Superman" (1993-1997)?