Initiated by the discussion on Russian dialects or the lack thereof, I researched the history of Russian a little and found an interesting text on the subject on the internet - it is written in German, so I'd like to subsume its content here.
The Slavic languages form their own branch within the family of Indo-European languages, and speakers of the original Slavic language began to settle in the Dnjepr / Vistula area at or shortly before the beginning of our modern reckoning 2000 years ago. Within the next centuries three main branches of diverging dialects developed, and the Eastern branch became the forefather of Russian.
In the 9th century Old Russian was the language of the newly formed Kiev Rus. In 988 this nation adopted Christianity, and bibles were imported written in a standard language called 'Old Church Slavonic'. This language was a kind of common international Slavic language used by all three separate dialectal groups.
The speakers of Old Russian retained their common language, the language used in the capital Kiev, until the 14th or 15th centuries, when it branched off into Russian, Ukrainian and Byelorussian. Church Slavonic as a common conversational language, apparently much like Latin in European Humanism, remained effective into the 18th century, when it was reduced to usage in church.
The period from 15th to 17th century is the period of the Moscow Rus, in which three main dialectal branches of Russian existed, the northern, median and southern branch of Great Russian. The median branch was under the influence of the dialect of Moscow, and this became the dialect used in literature. It differed from the others in some features, especially the pronunciation of unstressed o as a, the reduction of other unstressed vowels such as я as i and a couple of others which are present in Russian today. During that period, Russian lost three of its four different past tenses for verbs, apart from the remaining tense today it had the same ones as German and English have plus Aorist. It also lost the dual in between singular and plural. Around the turn from 17th to 18th century there was a heavy influx of foreign words, especially in the areas of science, military and administration, primarily from German and later French.
The various later reforms of orthography and discussions about grammar and style, especially regarding the difference between spoken and literary language, show that there was a relatively early urge to unify the language for all speakers of it, which may explain why dialects play such a small role. The political developments of the 20th century may also have had their share of influence in streamlining the language.
Robin