It's a number of things and I completely agree with most of the above answers, though some other factors should be mentioned. After Star Wars appeared, every movie has to be a blockbuster. You got a lot of good movies out of Hollywood between the lifting of the Hays code in 1968 and the time Star Wars came out and to find that quality you have to look at foreign or independent movies. They would never do a character piece or something based on a novel (unless it's also popular fad garbage like Twilight) like "One Flew Over the Cuckoo's Nest" after Star Wars because it would not bring in hundreds of millions of dollars. Things become formulaic and what worked before will probably work again and all other ideas need to be ignored because they are risky or the dollar amounts are too small even if they are profitable.
This works well for a while in the hands of competent people who know what the majority of people want and who are allowed to work without creative or political restrictions., which is why you still get good films in the 80s, 90s, and even through the 2000s. I can think of several excellent movies from almost any year of those decades. However, Hollywood is always at the forefront of "representation" both in the stories they tell and the production itself. Stories about women, queers, black people, and disabled people have to be told by queers, black people, and disabled people whether or not they can actually write them, and even when the stories involve cisgender, white, straight men, they have to be written and produced by someone else.
