Because they weren't idiots.
Not to be harsh, but when a critic, either a film critic or book critic, uses the same standard for John Grisham as they do for Ian McEwan, it's clear they're in the wrong business.
As a novelist who writes genre fiction, (in my case, dystopian urban fantasy) my goals as an artist are very different than someone like Donna Tartt or Ann-Marie McDonald. Now I would not put myself in their class as a writer, but someone like Brandon Sanderson, for example, who is probably the most popular epic fantasist working today, along with George RR Martin, are every bit their equal in terms of art.
One will sell millions of books. The other may win a Pulitzer. And both should be considered great novelists.
In the video below, taken in 1983, crusty theatre and movie critic (for the National Review, shocking) John Simon rips Return of The Jedi and any critic who enjoyed the movie. What's problematic here isn't that he didn't like ROTJ, probably the weakest of the first three films, but that he applies the same standard to all films, as if every artist is trying to achieve the same goal. This is the kind of snobbery that drives arrists crazy, because it suggests there is only one worthwhile form of a certain art. It suggests purity.
And it's perfectly ridiculous.
That's not to say that we can't criticize genre fiction, in whatever form. Sure we can. There is great blockbuster film making, like The Dark Knight, and bad blockbuster film making, like The Postman. But in the very least, a critic should have the sense to judge the art, at least partly, on what the artists is attempting to do, not some invisible ceiling where there is only one form of acceptable artistry.
Here's the clip. And if you ever wondered why Roger Ebert was so popular, here's a hint. Because he got it.
Yeah, I miss him, too.