This is asked by Dan Allen in his blog post Red Sox Nation. He links to this on Facebook with the lead-in, "why the Red Sox are awesome and churches are not."
Dan makes the point that Red Sox Nation (as a biased Giants fan I could re-write Dan's post with my team in view) looks more like what Christianity should look like than Christianity does. Most of the churches I have attended have preached that "it is Jesus, not Jesus plus something else," yet they have insisted that I must become a formal member of their church or something close to that in order to avoid having my salvation questioned for not doing so. Why are Christians marginalized for simply wanting to be part of Jesus' followers?
Quoting Allen's two money paragraphs:
The other element of this is what exactly makes one a member of a particular church? To be a member of Red Sox Nation you need simply love the Red Sox. When you go to a game at Camden Yards you are essentially a member of Red Sox Nation gathered at Camden Yards. When you go home to Boston later in the week and go to a game at Fenway you are gathered with Red Sox Nation there as well. Are you more officially a member of either gathering? Are you disloyal to either for going to the other? Of course not. The more places you go, the more games you gather at the more you show your love of the team, the defining trait of being a member. I mean, hell, when you go down to the local sports bar to put down a pitcher of Sam Adams Boston Lager and watch the Sox destroy the Rays, you are a member of the gathering of Red Sox Nation there too!
Churches don’t work exactly like this. You aren’t really allowed to just be part of the group that you are around. You may be allowed to be a guest if you visit family out of town and stop by their church, but if you don’t gather with one group consistantly week to week at the designated meeting place, you are considered a church hopper and somehow, unlike going to all the Red Sox games wherever you can making you a bigger fan, this somehow makes you a less mature, less serious Christian.
Dan makes a good analogy, and I encourage you to read his full post to gain the full flavor of his argument.