That's not "turning into Russia 2: Funny Accent Edition", though. That's more like that "melting pot" kind of thing that some Americans love to talk about. There are a lot of different sub-cultures under a single unifying label of "Russian", it's not like there are some "how you should behave if you're a Russian" guidelines that define a Canonic Russian Culture. There just isn't one, at this point, after 15-16th century at least. Russia is way too big and diverse for that to be a thing.
Oh. It is a melting pot, violent edition.
Russian situation is not unique. Look at our Spanish friend with his dislike of Catalonia. (Really his arguments are so funny for me, I can find a matching Russian for most of them)
Before WW1 English, Turkish, Austrian, German and other assimilation machines were roughly similar to Russian. It is after WW1 when real shit started. Some empires died, Some became weaker and more liberal, Some went exterminate other nations route, Russian Empire transformed into USSR that was much worse than Russian Empire.
USSR never bothered with turning entire population in Russians. It worked on transforming its population into a brand of two legged cattle called "Soviet people". Russian culture and language were tools in this process not the goal. When this tool didn't work, genocide was used.
Actually karma is a bitch. Real Russians have rather unpleasant lives nowadays as they have
no own country. Their lands are populated by a weird mix of people who call themselves Russians but if you take away the language, there are very little Russian in them. They are rebranded Soviets with their awful hypocrite morals.
I thought most native languages have been erased because they were less competitive than Russian one and thus nobody wanted to speak them? There surely isn't any "concentrated efforts" on pushing Russian language in Ukraine, for one, and almost half of the country speaks Russian!
Inertia. And pro-Russian parliament for all that time. Parliament that never did anything to reverse the directions set in previous centuries.