I asked a question around a year ago regarding how Russia got to its extreme size and I thought this would make a good follow up question. How is it that Russian culture was able to entrench itself in such a large area when to the South of it, dozens of countries exist in roughly the same equivalent area with vastly different cultures and beliefs?
So little ethnic diversity? Russia is home to over 160 different ethnicities and indigenous groups. Of course ethnic Russians are by far the biggest group, but ethnicity is very important in Russia and still remains an issue in some places like the Caucasus, and in the major cities which have recently experience an influx of Central Asian immigrants.
Russia is very different depending where you go, especially the south of European Russia, which has a very high concentration of Muslims. There are even indigenous nationalities like the Kalmyks and Buryats that are Buddhists.
A lot of these nationalities have their own languages, and while some of the smaller ones are dying out, others such as Tatar and Bashkir still have relatively many speakers.
It is true that Russia used to have more diversity, but this regional diversity decreased as ethnic Russians spread further to the east and assimilated smaller, isolated populations. The Orthodox church forced the conversion of many tribes and opened local schools, which, of course, taught Russian. Russification continued under Stalin, who made Russian mandatory in schools throughout the Soviet Union.
The other, continuing reason behind Russification is that western Russia still dominates in terms of politics, economics, and popular culture. Much as Castilian Spanish has been common language between the various groups in Spain, Russian is official and by far the most common language in Russia. As I mentioned earlier, ethnic prejudice might also play a role because many Russians see non-Russian ethnicities as socially and even racially inferior.
Hope this explains it. Sorry for the wall of text.