I agree that most of these films were too dark to win Best Picture. However of the five nominees you listed, only The Exorcist should have taken Best Film. In all four cases, I would have voted for another nominee:

1971: The Last Picture Show. A haunting brilliant film that introduced us to another legendary Director and some of the greatest acting talents in history Among them, Cloris Leachman, Ellen Burstyn, Jeff Bridges, CybilL Shephard and Timothy Bottoms. And it was dark too in tone.

1980: Raging Bull. If anything this film is darker than Elephant Man with less optimism by the end. Scorcese officially become the greatest director working, Robert DeNiro became the greatest actor of all, and we all got frustrated that Joe Pesci would only seem to work every ten years.

1994: The Shawshank Redemption. imdb.com currently is considered the greatest by its voter, and while I think that's overstated the fact, I can't deny that this is one of the greatest films in history. At times, its as dark as Pulp Fiction, but not as flashy, and it ends with real hope not one that's messy.

2019: All right, I almost agree with you on Joker. Extraordinary film, a true look into the mouth of madness and Joquin owned the screen. But there were a lot of films I would have rather have seen win: Greta Gerwig's Little Women, JoJo Rabbit- our generation's Producers - and yes Tarentino's Once Upon A Time In Hollywood. There were a lot of good films that year.