Ah so many takes of "The audience must be wrong!"
Always enjoy those.
Or maybe he created a film that appeals to an extreme niche
Sorry, but it's my experience with all types of art that the general audience is usually wrong, at least when taken as an overall average like this. This is especially true of mass-marketed films. Perfect example is Eyes Wide Shut, which was marketed as "sexy Tom Cruise/Nicole Kidman thriller" and has a CinemaScore of D- despite being a masterpiece. I don't blame Stanley Kubrick, or the studio, for the audience response. I blame the audiences who were incapable of evaluating art on its own terms rather than what they "expected going in."
As far as appealing to an extreme niche, the question we should be asking is "why does it only appeal to an extreme niche?" rather than "why did he make this film this way?" or "why did they market the film this way?" Why does the marketing have to shoulder all of the blame here? When talking about mainstream audiences,
why isn't this a movie for them?
In short, I think the problem lies with people who expect the marketing and trailers to tell them
exactly what they should expect to get when they walk into a theater. Like it's a product made out of plastic rather than art. And then we wonder why so many trailers nowadays reveal nearly everything about the plot.
I realize that this is very much an "in a perfect world..." way of looking at things. I've just never been one to "expect" anything when experiencing a new work of art, and I don't understand why so many people do.