ECONOMICS. Thats the one word answer why the world series doesn't include other countries.
By your premise the Tony Awards should be the "World" Tony Awards and open to all first class productions which play in the major cities of the world....the Oscars would eliminate the "Best Foreign" film category and let all films compete for Best Picture regardless of country of origin.
Keep the World Series U.S. centric and let's be thankful for one less element of globalization.
Toronto was champs for a while, but as for the rest of the world, if the players are any good they're gonna wind up here in the North American Majors eventually (Pedro Martinez, Hideke Matzuie (sp?), and all those Cuban guys).