From what I’ve read of the review of the ‘Affirmative Action Empire: 1923-1939’ by Terry Martin:

Martin significantly advances our understanding of the early, formative years of Soviet nationality policy, providing a subtle and lucid reconstruction of its unique conceptual underpinnings and its stormy evolution. Contrary to earlier Bolshevik mantras, Lenin and his partner in nationality policy, Stalin, committed the {union} by 1923 to developing non Russian languages, elites, territorial units, and cultural forms-all at the expense of Russian nationhood and culture. Hence the Soviet Union became, in Martin’s odd phrase, the first multiethnic “affirmative-action empire.”

That’s good, but then there’s this next part

Then, in the period of the Great Terror (1933-38), the experiment ended. Russian nationality and culture were revived, and “bourgeois nationalism” replaced “great-Russian chauvinism” in opprobrium

Why did that happen in such a manner?