My first observation is that America is a very homogenous culture. From the perspective of writing these notes this fact is hugely convenient because it facilitates and justifies a whole load of other generalizations I will go onto make later. But first let me explain this point. There are obviously big differences between say the highly conservative Americans of the Carolinas, and the relatively liberal Americans of say California. And it is also true to say that politically America is a highly polarized country with lunatics representing every extreme and implausible viewpoint, as well as many of the viewpoints in between.
Yet at the end of the day, Americans from every corner of the country can actually understand each other in speech. In the one year I spent in Scotland, the phrases I used most often were, "Pardon", "What", and "I’m sorry I don’t speak that heathen Gaelic dialect, could you say that more slowly, and maybe use a bit of English". Of course it is not just the Scottish who have difficulties making themselves understood, I was no more successful at making myself understood to the average Scot, than they were to me. And similarly strong regional dialects exist throughout the country. My explanation for this difference, and also for America’s homogeneity is based on a simple application of Darwinian Evolution to Language. This is obviously a valid application of Darwinian evolutionary principles as language is a continuously evolving entity. Over time organisms (humans as well) evolve, and just as the finches on the Galapagos islands evolved different adaptations to suit their different environments, so I reason it is with language. The differences arise over time in cases where breeding populations are separated (or at least interaction between breeding populations is minimal). Given that, for instance, most Welsh people wouldn’t spit on an Englishman if he were on fire, far less talk to him, the scene is set. A Welsh friend once explained to me the reason the Welsh language has no swear words is because the only time a Welshman needs to swear is when he is talking to an Englishman – and then they do it in English.
The key difference to the history (and thus the evolution) of America and Britain is of course that Britain has a history while America has, well, for the sake of politeness lets call it, "a recent past". In San Diego, the old town dates back a 100 years, although it has been twice relocated. It is less an old town than a museum. British history is officially recognized as starting with the arrival of the Romans in 55bc, although English civilization existed long before then, for instance Stonehenge was built 2 millennium before the Roman's arrival. In 1505, fifteen years before the Pilgrim Fathers arrived and the real start of the current American history, Guy Fawkes was attempting to blowup the British Parliament.
Thus I reason the lack of American history is the major contribution to the lack of diversity in the American population. American history was still in its infancy until the invention of modern transport and modern communications happened. Thus individual cultures were not isolated, and thus didn’t develop the same strong cultural divergence.