Money leaving russia means that it's currency is weaker. If it did not have a currency (instead used bitcoin) then prices of goods it imports and exports would not go up and down based on the state of it's economy.
This argument doesn't really hold water until the price of Bitcoin stabilizes though. No?
If your local currency is almost certainly going to fall by 20-30% due to upcoming war and economic instability, wouldn't you rather keep your money that only has a chance that the price will go down by 20%?
Another issue is control. If you have your money in fiat then the government could potentially seize it. In times of war seizure of money is more common as the government needs to pay for it's war machine.