Citizen Kane was a revolutionary development in film. Prior to it, movies were filmed like the camera was a spectator at a play. Welles put the camera into the scene in
Citizen Kane. Compare it to two of the great films that came before it,
Gone with the Wind and
The Wizard of Oz. The narrative also seamlessly used flashbacks.
We don't really notice these things today, but they were new techniques at the time. I agree that the story is the weaknest part, but it was good enough to win an Oscar.