The short answer is that many people didn't get the movie when it was first released.

The movie is of course very different from the book.

In the movie it's clear from the start that both Jack and Wendy are in a bad place.

I think people are more sensitive to picking that up today, then in the late 70s / early 80s.

The book is about alcoholism, the movie is about mental illness and controlling behavior/gaslighting.

Back then, movies/shows about mental illness were far more on the nose, so people just didn't understand the movie.

And of course the book leans far more into the supernatural aspect.

Kubrick understood the essence of the story. I think King was too close to the subject matter, to fully understand what he had written.

King identified with Jack and wrote Wendy as a perfect wife.

Kubrick understood that Wendy should be the main character.