Written languages in general
Written languages are not vernacular languages. Written languages tend to be more formal, more refined, more stable, more standardized, and regarded with more authority than the spoken languages they co-exist with.
People learn their local spoken language from birth, just by picking it up. Written languages are normally taught in schools. Whatever the children speak, learning the written language gives them access to a tradition of written literature stretching back for centuries, and makes them part of the wider world of all the people who share that written language, regardless of local dialect. The larger the number of people who use a language, the more important the written dialect becomes, because of the impossibility of standardizing pronunciation and regional vocabulary. Because writing stays put on the page, unlike speech which disappears into the air, the written language tends to be more stable than spoken dialects, and also "anchors" the rapid variation in the spoken dialects to some degree.
Many written languages do not even reflect spoken language very well. For example, the same written Chinese, with relatively few local variations, is used throughout China, despite the great variation among the Chinese languages, some of which vary even in grammar from written Chinese, not to mention Japanese, which uses Chinese characters and has a grammar utterly unlike any Chinese language.
English is no exception. To illustrate, there are numerous regional dialects throughout Britain, which include vocabulary that has almost never been written down. For example, in Yorkshire dialect, "nowt" has meant "nothing" for hundreds of years (though it's now fading), but it's almost never found in print. In writing, you would write "nothing".
Scots dialect was occasionally written, but usually with an "apologetic apostrophe" to indicate where it diverged from standard written English. Much of Scots dialect lacks standard spellings, because people seldom made any attempt to write it. In person, I've sometimes found Scottish speech nearly incomprehensible, but I've never had a problem communicating with Scots in email. There is a recent movement to write in Scots. They're trying to make a variant of written English to correspond with speech—and they've already run into the usual problems. The Scots Wikipædia recommends that "fowk uises 'tradeetional' pan-dialect spellins."* In other words, in order to make Scots into a written language, they need to find a common denominator across regions (of Scotland), and they're treating older, traditional spellings as precedents to follow in order to create that standardization. That's how written languages tend to work in practice.
English in the United States has much less regional diversity than in Britain, but there are definitely vernacular dialects in the United States. AAVE is spoken nationwide, but, like Scots dialect, has seldom been written and lacks standard spellings. It's a true vernacular. Regional terms in the United States often cover such a large region that people don't know they're regional, so they go more often into published writing, yet even so, the pressure to standardize often leads to a single word's dominating in print: for example, look what "seesaw" has done to its regional competitors. Words unique to smaller regions seldom see print. For example, compare "papaw" and "grandfather". When regionalisms do appear in print, in stories, poetry, diaries, proverbs, dialogue, advertisements, etc., they usually impart a regional "color", because they diverge from the trans-regional standard and readers are expected to notice.
Dialects of written English
The "user base" of English is so huge, however, that the written language itself comes in several dialects. English has on the order of 400,000,000 native speakers, in several countries spread across more than half the Earth. Another 800,000,000 speak it as a second language. It's an official language of many countries where people don't even speak it as a mother tongue.
One reason why English has such wide use among so many peoples is that it has a pretty well-standardized written form. The main reason, of course, is access to international trade, but without a standard written form, English could not enable such far-flung usage and communication across national boundaries and regional dialects.
Yet there are distinct written dialects, though the differences are (to me) amazingly small. The biggest difference in written dialects, of course, is the British and American spellings: "colour" vs. "color", "programme" vs. "program", etc., though I understand that even these are showing some tendency to standardize. Standard Liberian English has a distinct written form: for example, "President pro temp" instead of "President pro tem" (technically, that's a distinct form of Latin used within English but it's still a regional English variant). In some cases, there just isn't any common term: for example, "bonnet" vs. "hood". In those situations, one is forced to write for only one part of the English-speaking (or rather, -reading) world.
The English Wikipedia tries not to favor any one national variety of English. Since there is no one international written dialect, different pages get different national varieties. In practice, despite the national differences, most of the conflicts tend to be stylistic, not endangering intelligibility. To see how Wikipedia deals with national varieties, read here. The pressure to standardize competes with the pressure to respect national differences, and usually wins—but not always.
* Shouldn't it be "pan-dialeck"?