Why do we call New York the “The Big Apple?”

During the 1940s, Robert Emmerich, who played piano in the Tommy
Dorsey Band, wrote an obscure song called “The Big Apple.” It was
soon forgotten by everyone except legendary reporter Walter
Winchell, who liked the song so much that in his daily column and on
the air he began referring to his beat, New York City, as “The Big
Apple,” and soon, even though Emmerich’s song was long forgotten, its
title became the great city’s nickname.