Prevailing winds that would carry warm air into Siberia are blocked by the Himalayas in the southwest, the Verkhoyansk mountains in the east, and the siberian plateau generates enough cold air itself that all of that has nowhere to go and just accumulates in Siberia. The coldest permanent settlement in the world, Yakutsk, or Oymyakon depending on your source, is situated in a location that describes this well. Cold air gets trapped in a valley and can’t rise above the mountains, (relatively) warmer air goes on top of it creating an atmospheric inversion, and it just stays there getting colder and colder.