me_irl

There's actually a good reason for this! Explained in the TV Tropes page about why Seinfeld is Unfunny - basically that so many pieces of art were so revolutionary at the time, they they have been endless copied and reiterated over and over, so that modern audiences seeing the original piece of art don't see it as anything special.

Citizen Kane is called out specifically as:

Citizen Kane, oftentimes trumpeted as "The Greatest Movie of All Time," tends to inspire "what's the big deal?" responses from modern viewers, especially since Post Modern movies have become the norm and the cinematography has influenced so many other films. And everyone knows what the twist at the end is.

Citizen Kane was also, in the context of Hollywood at that time, a big challenge to the cheery sugar sweet Hollywood stories. It was critical of the idea of The American Dream and the notion of "success", namely that a man who is outwardly a public success like Kane could still be a failure in terms of personal ambitions and relationships. Its refusal to tack on an unconvincing Happy Ending similar to earlier serious films made it far harsher than other movies of that time. Subversions like this are much more common these days. The narrative structure where we see Kane at different parts of his life, all of them intercut with each other rather than following a straight chronological pattern from childhood to old age, was cited as an influence by the more radical film-makers of the 60s and 70s who enjoyed playing fast and loose with chronology.