This will not be one of my more popular posts, but I think a lot of people pretty much know the answer anyway even if they will not admit it.
Right now there is a growing trend towards globalization, and the unification of humanity under one world goverment. No independant nations or cultures, and no exceptions. This is mostly happening due to the spread of ideas, and while there is a lot of resentment, it's happening slowly but steadily. One of the reasons for the internet blockades you hear about isn't to stop "child porn" or whatever, but for patriots and nationalists to limit outside ideas coming in in an attempt to slow or stop the process. War and conquest WILL enter into the equasion eventually, but I expect the majority of globalization will happen peacefully and at a glacial pace. Nobody is going to like tear down their flags in the next six weeks, when it gets to that point it will seem like a natural evolution of what is going on.
At any rate, the big "winners" receive some resentment for this. A common "whine" in the global community is for second tier and lower nations to scream about the increasing trivialization of their culture and so on. Pretty much everyone hoped that their culture, history, and way of thinking would be the one the world would unify under but it's generally not happening and this kind of a reality check is unpleasant.
Right now there are only so many nations that are a real "factor" on world affairs in of themselves. The US, China, Russia, etc... all are big players. While "lesser" nations like to assert their independance (especially a lot of America's allies who maintain more freedom than those who become puppet nations or get added to a bloc), the bottom line is that pretty much everyone has an eye on what is going on in these places to an extent. The entire world watches the US elections and inserts input for example, yet most nations would have a hard time telling you who is in charge of many other countries except for the US.
The communism thing pretty much comes down to a disagreement on how people can be managed on a large scale, and how who winds up on top, and among the masses at the bottom is going to be accomplished. It could be analyzed back and forth on a lot of levels, and really isn't important. The bottom line is that both Russia and China preach Socialism/Communism and are in direct philisophical opposition to the US. Russia has disputed our ideas militarily in the past, and China is currently working on developing the power to do so (ie building a substantial navy and airforce to get it's massive population and military yield out of landlock). In addition there is constant drama among the major powers over technology. For most people there is an assumption that nukes prevent World War, but frankly defensive technology is getting better and better, and each one of those big three has demonstrated some abillity to stop incoming missles. US defense batteries caused treaty crisises with the Russians as have our placement of defensive bases, China has shown the abillity to blind some our our satellites (and has stolen some of our military tech), Russia has so far claimed to have similar systems but to my knowlege has yet to demonstrate anything. I don't doubt they think they have something, but being the USSR it's a craps shoot how good it is.
At any rate, the point is that only these big three nations (and maybe eventually India) can influance anything globally. So basically if you want something in a game that represents a global threat it has to involve those powers. In general the US pretty much runs most of the western military and global peacekeeping forces. We provide the backbone of global manpower, and for all complaints aside, it's usually our logistics and administration that winds up running any kind of multi-national task force. If you wanted to stop The Western World's military on some massive level hitting US bases like The Pentagon would be the priority target. Sure individual nations might still be able to run their own military, and maybe cobble things together at some point from the chaos, but that's pretty much where "The Free World" is run from, all resentment aside. This is also why we're pretty much the "bullseye" for The Middle East for example.
In general "OMG! they are invading New Zealand" just doesn't have any kind of global implication. What's more as long as the US was still around, there would be a coordinated global response to that. So basically before something like that would be likely to happen, they would at least try and take out the US or at least the command infrastructure we provide.
While unpopular a lot of people understand this on some level, which is why a threat to Washington can translate so well in a global marketplace. What's more most of the world KNOWS DC is the Capitol of the US, and knows who The President is, as well as other things that are located there.
On a less global level, the US doesn't feature into ALL games. There are plenty set in Europe and such especially when it comes to horror games, adventure games, and similar things. The thing is though that if you pick a country like New Zealand, which might be really nice and all, and set a game there, 99% of the world is going to look at it and just not get it. The proliferation of US media and such means that most people globally are familiar with say California, New York City, and similar places even if they haven't been there. Toss out some town from New Zealand and they are going to be totally lost. This of course comes down to globalization and the spread of ideas though. The massive international success of syndicated TV shows from the US and Britan (BBC) have lead to the kind of familiarity that promotes development on a large scale. With China for example cities like "Hong Kong" are pretty well known cinematically as well due to the "Hong Kong Action Cinema" if nothing else. For such a small nation Japan also fought a VERY long and VERY hard battle full of much mockery to enter itself onto the world cultural stage. Japanese stuff set in Japan can succeed as well as it does because of a lot of effort put into promotion. All those Japanse Horror Movies, Anime flicks, and Samurai Melodramas had a cumulative long term effect. I suppose New Zealand could do the same thing (maybe) if it worked at it for decades and was willing to deal with a lot of mockery for a very long time in the "hey, I hear it's really big in Japan, lulz..." vein.
I guess what I'm saying is that this is what globalization is all about. American culture is becoming one of the global cultures, and quite probably one of the biggest (if not THE biggest) influance on the planetary culture we'll one day see if our species is going to survive and move out into space or whatever.
All resentment aside, as "interesting" as someone's culture is to themselves, when dealing with the mass market and things like games appearing on anything other than a local market, the problem is most people around the world aren't going to be able to associate with it.
Maybe it's a bad example, but if you were to say try and market a New Zealand-centric game to say eastern Europe a lot of people are going to say "WTF" and just not associate much with it or "get it". Ditto for if you took something seeped in their culture and sent it to New Zealand. On the other hand you can take a game involving America and people in both places would instantly recognize "Washington DC" and why an attack on it by anything from foreign powers to space aliens would be a very bad thing for pretty much everyone.