Keep in mind that the US was settled relatively recently, and most American and Canadian accents are descended from a single dialect in south-western England (the only exception I know of being the dialects in New England, which have a strong Cockney influence). There are some regional differences, but not enough time has passed to give English in the US the same variation found in most languages. Take away technology and add another few hundred years, and distinct languages might start to emerge across the US, but there hasn't been enough time for that to happen yet. With modern technology that might never happen at all Despite being much smaller than the US, there is a much bigger variety of accents in the British Isles. I strongly suspect the reason is simply that they've had more time to diverge, and there were already a lot of accents and dialects there before Europeans discovered the Americas. With modern technology, the differences are disappearing. I've found that a lot of teenagers and young adults in England have a noticeablely stronger RP influence than people their grandparents' age. I've heard Americans, Britons and Australians use slang (spread through the internet) that comes from the other two countries. In Australia and Britain, there is an increasing tendency for younger generations to use traditionally American pronunciations of words. I worked in adter-school care for a few years, and I noticed a lot of small Australian kids say words like "glass" and "can't" with the vowel in "cat", probably something they picked up from American cartoons. English has a lot of different accents and dialects, but they're rarely different enough to cause misunderstandings. If you put a bunch of native speakers from all over the world in the same room, you'd be able to tell where each person is from from they're accent; but they'd be able to talk just fine without having to switch to a standard English (which doesn't exist and doesn't need to).

Yes, we can understand each other just fine. But there are some regional/subcultural accents that are stronger than others, and also some groups that are far away from each other. I imagine if they met they would have a hard time understanding each other, but they are unlikely to. Most of the difficulty comes within the various immigrant communities communicating with native speakers and with each other.

Thanks a lot for your reply. But the territory of the USA is so enormous. The pronunciation is different, I suppose. Would you recommend me a website, where I can find some exampeles?I explore the Americans dialects and it's very important for me to realize what the real problems arise by communicating with each other.