Ah, yeah I was thinking of brand and identity differently.
I think that's probably true for a number of reasons. The first is that America hasn't really had a supporter's culture in any of its sports. There are fans, but the notion of fans having a formal group, a formal seating section, etc. isn't there in any other sport here. Most American fans talk about their connection to the team through family, meaning their literally family (i.e. I went with my dad to the stadium) whereas I get the sense that in England that family is broad enough to include the SG.
Second, I think the fact that in the US you only have one team per city has a lot to do with it. Whereas in England, the different breakdowns within the city often reflect not just geographical but political, religious, economic or other differences. There's no rivalry in the US with the connotations of Real Madrid v. Barcelona. Most of our rivalries break down into "dominant team v. underdog."
Finally, I think Americans really like winners. Particularly with many cities boasting multiple sports teams, if a team is a loser they can quickly fall into obscurity hurting the bottom line and their prospects of attracting future players. Without that same social aspect, winning is really all that matters in the professional ranks (This is part of the reason promotion/relegation would have a really hard time in America).