Finally "figured" it out! Thank you, Google :)

Question: Why do we refer to NYC as "The Big Apple"?

Answer: John J. FitzGerald, born in 1893, was a horse-racing writer for The Morning Telegraph in the 1920's and was the first to popularize the term 'The Big Apple.' While on assignment in New Orleans, FitzGerald overheard African-American stablehands refer to New York City race-courses as 'The Big Apple.' FitzGerald loved the term so much that he named his racing column 'Around The Big Apple.' The title to the column became synonymous with the New York City racing scene.

A decade later many jazz musicians began calling the City 'The Big Apple' to refer to New York City (especially Harlem) as the jazz capital of the world. Soon the nickname became synonymous with New York City and its cultural diversity. In the early 1970's the name played an important role in reviving New York's tourist economy through a campaign led by the New York Convention and Visitors Bureau. Today the nickname 'The Big Apple,' which replaced 'Fun City,' is the international description of our city and is synonymous with the cultural and tourist attractions of New York City.
Therefore, it is only fitting that the southwest corner of West 54th Street and Broadway, the corner on which John J. FitzGerald resided from 1934 to 1963, be designated 'Big Apple Corner.'