There is a high cost for the Russian government to do that - they have to organise some thugs, analyse the blockchain, set up back channels to on-sell the crypto, etc, etc. With the ruble they print more. Maybe update some numbers in a database. Very low cost. The harder it is to execute, the more likely it is for families to preserve some wealth.

And they can skim off the top of all Russians at once. Even bitcoin is a pretty uncontestable step up - fiat currencies are bad for storing wealth at the best of times but this is really not the best of times. This is the time to flee a sinking ship.