Because he murders and jails all opposition, part of having free and fair elections is to allow political opposition to exist.
If he stops murdering people that oppose him and he allows free and fair elections, there would be more people to actually run against him.
Russia isn't a real country, its just Putin fiefdom, Russia exists only to serve Putin's interests, if you are a Russian nationalist or a Russian liberal that only wants the best for the country, it doesn't matters you oppose Putin and your life is forfeit.
That's a mafia State.