I'm not certain that there is an answer to this one:
Americans refer to our teams as The
Example: The New York Yankees
The British in my experience do not.
Example: Manchester United
I know that occasionally the British will throw in a the, such as The Arsenal, but I believe this is a nod to the actual Woolwich Arsenal.
We also tend to pluralize our teams, where I don't believe the British do this typically. (Not certain on that one . . . A bit of help from across the pond would be appreciated.)
Could the pluralization be compelling the usage of The?
Of note, the only time The gets truncated from an American team name is when it is used as an adjective. e.g. Legendary New York Yankees pitcher Mariano Rivera retired this past season.
Best Answer
As David M suggests it is due to pluralization.
Americans tend to name their teams in reference to the collection of players on the team as a group. "The Yankees" or "The Red Sox" references the collection of players and managers who make up the team. A player is a Yankee, or a Red Sox, and the collection of players are "The Yankees".
European football team names tend to reference the club as a single entity. "Arsenal" is the name of the football club, the club is considered a single thing that exists independently to the collection of players, staff, etc.
But people will often refer to "the gunners", in reference to the current Arsenal players of the team when they want to talk about the collection of players.