The 1904 World Series was never played. There was a business dispute between the established National League and the upstart American League.

However, the primary reason was a petty feud between New York Giants manager John McGraw and American League president Ban Johnson.

In 1903, the Boston Americans beat the Pittsburgh Pirates in the first World Series. In the offseason, both league's agreed that their respective champions would again meet in October.

Despite the agreement, Giants owner John T. Brush publicly stated as early as July 5, 1904, his team would not play any champion from the American League, which he and McGraw derisively called the "Junior Circuit". At the time, the New York Highlanders led the Americans by one game. Boston won, but McGraw said that his team was already the world champion because they won "the only real Major League." The Giants also deemed the World Series an exhibition with no real value.

The Giants were mercilessly criticized in the newspapers for "chickening out" on facing the Americans that they agreed to play the World Series in 1905, where they beat the Philadelphia A's in five games. Not long after, an agreement was made and the World Series became a permanent event.

The Series has been played every year since, except in the strike-shortened 1994 season.

More Info: en.wikipedia.org