That's what I get for rushing a reply and not supporting it well before driving my wife to an appointment.
I switched allegiances in my NBA and NFL teams because teams were added in my home market, which was something I always wanted as a North Carolinian, since there were no real local teams. If that makes me a bandwagon fan, so be it, but I've stuck with the teams in the North Carolina market, despite their lack of success.
I was a general ACC fan in college sports until I decided I wanted to go to Duke and have supported Duke teams ever since. Don't call me a bandwagoner when I've put up with Duke football for the last twenty years. What allegiances I've switched have been due to major changes in the leagues, or in my life. In pro sports, I've seen "my" team win two championships in all my life, the 1983 76ers and the 1995 Braves. I've been more fortunate in college basketball.
I don't think allegiance has much to do with why Americans haven't taken to soccer/football/futbol. I think it's more the American style to take to American things, and while one can debate the origins of baseball, football, and basketball, they were all popularized in the United States. Soccer was not popularized here, and has had a harder time breaking through. I personally think soccer is probably a "purer" sport than any of the more popular team sports in the United States. The objective is so simple. Eleven players are trying to put the ball into the opponents' goal while trying to prevent their opponents from doing the same. It truly is beautiful in its simplicity. I know there are nuances far beyond that, but the objective of the game is so pure. As I've gotten older, I've grown to have a greater appreciation for just how difficult it is to put that ball in the opponents' goal, and don't mind the low scores a bit.
Most American sports enthusiasts have the impression that the world is calling them stupid or uncultured for not loving the sport as much as those from other countries do. As a result, many get defensive about that and point out what they see to be shortcomings of the sport, and we get these standoffs over the issue. People don't like being called uncouth, and people don't like others disparaging the game/sport they love. Both these things are completely understandable. I think with demographic shifts in the United State and more youth participation, soccer will continue to slowly gain ground. I don't see MLS ever being as popular as the NFL, MLB, or the NBA. In southern states, it probably has a shot as being as popular as the NHL at some point. It's a more TV-friendly product than the NHL, when those damned horns aren't blowing.
And another thing Americans don't get about soccer are the riots. We just don't get these stadium riots at all. The American way is to watch sports at home, and go riot in the streets.
First up, and I know this isn't tofu's point, but other sports in the UK. Rugby is huge, but only in certain areas (South West and North-East-ish). Cricket occasionally (although at non-national level, not all that huge). But yes, football is the biggest thing by a long way. We're more interested in non-localised stuff like Formula 1 (where the last two world champions are English).
Back to the point, changing allegiance in sports teams is incredibly rare. Size of country may make a difference, as people can move to the other side of the country and it's still only a few hours journey to see your team. I don't know. I don't think there's anything unique about British DNA, or anything.
Re: the world calling the USA stupid or uncouth, every single time I've seen this happen, it's been the person from the USA who's started the argument by saying "football's rubbish, why do other people like it?" I'm casting my mind back, and I can't come up with a single example of British people criticising the American attitude to football first...simple explanation is, we just don't care if you like it or not. Much the same as your attitude to us and baseball, basketball, ice hockey and American football, but those arguments never start because we don't start them off in forums like this.
I'm prepared to be proved wrong about this, but look at the World Cup thread - things were fine up to thunderbolt weighing in with "I've yet to meet anyone who gives a shit". Then why fucking post in a World Cup thread???
And we've not rioted for a fair old while. We're quite polite these days.