As it stands today, the law requires there to be a stamp from security on each boarding pass and that is the reason why they require paper. I am not too familiar with SU, but S7 has those machines which scan the barcode on your phone and instantly print the paper boarding pass. In DME, the machines are located in the check-in area, as well as just before security.
As far as I know, by the end of the year, Russian airports will start accepting mobile BPs.