For context, Estonia is a NATO member with significant Russian minority population
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Main requirement frequently mentioned preventing the citizenship is unwillingness to pass the language exam. Same or similar rules (understanding the language, accepting constitution, paying taxes, etc) apply in other countries, including Russia. One of the issues is that Estonia does not accept dual citizenship and Russian citizens are often not willing to give up their passport even when they have never lived there (this attitude has changed during the last year, however).
To make matters worse, some people (66k in JAN 2022) do not have citizenship, they however have a special status in Russia, making traveling there easier than with Estonian citizenship.