That's not what I mean when people bandwagon. When a team does very bad for a very long time, it's understandable that the people in that region start to switch over to other sports, since there's not much to cheer about. But it's annoying when a fan of the Mets suddenly started playing with the Phillies because he thinks he would win if he played with the #1 team.
And you need to improve your knowledge surrounding Red Sox - Fenway Park history. As a Red Sox fan, I can tell you that large crowds attended home Red Sox games years before 2004. To outsiders, it may seem that the Red Sox have not been doing since they have not earned a World Series title in 86 years; in contrast, there are many years between 1918 and 2004 where the Red Sox did outstandingly well, but did not get the World Series title.
In fact, every game in Fenway Park has been sold out since sometime in 2003.