Logistically it's just difficult for fans of smaller nations to travel to every game so the games will be filled with Americans neutral to any team playing that day in the stadium. And I think that loses some of the luster that comes with the Euros or the world cup in smaller geographic areas.

It's kind of what makes the Olympics fun. Everyone comes to the host city. When the event takes place across 4 time zones geographically it's gets diluted.