Partly because the US government has said it will not ban bitcoin. Certainly, Putin does not want to be left behind in the race. Second, Bitcoin can help Russia avoid dependence on the US dollar. In the same statement, Putin stressed that if the US continues to impose sanctions on Russia, It is likely that Russia will abandon oil trading in USD. Currently they still trade oil in USD but apparently, he is implying that bitcoin could be used in the future to replace USD.
I know they said that but we still cannot know if Putin will use BTC instead of USD if that happens. My guess is that they are going to try the EURO first. Only if that also fails they'll look for an alternative. Right now EUR and USD are dominating the financial world. Luckily though, EUR won't survive if the USD fails because they are all connected.