Some time ago, prior to giving a talk on world building to some university students, I looked up the top 100 grossing Hollywood films of all time. 97 of them where Science Fiction or Fantasy. The genre clearly has appeal. Now Disney's latest Fantasy epic, "John Carter", looks set to rank among the biggest money-losers of all time. It had a great cast, sumptuous special effects, oodles of action and an entertaining story. On paper it would seem to provide the same sort of spectacle as Avatar – and, for a Hollywood blockbuster, spectacle is essential. So what went wrong?

I enjoyed it – but I think I went to see it for different reasons than most people would. I love classic Science Fiction and Fantasy. The story was never going to win any awards for originality – but that's because it was written by Edgar Rice Burroughs 100 years ago, and has been plundered by same genre films ever since. A guy is mysteriously transported to a faraway place, where he turns out to be "the chosen one", beats the bad guys and gets the girl. Just like "Avatar", you know how its going to end. And that's all fine. "Flash Gordon", "Avatar", even the "Star Wars" saga have all nodded their heads in its direction.

In terms of story "John Carter" and "Avatar" are very similar. An outsider arrives to save the native people from the bad guys and falls for the princess in the process. "Avatar" is stronger because we empathise with the "Poor Natives" who are in the process of being steamrollered by "The Evil Corporation", whereas with "John Carter" we are rooting for our hero to save a privileged, kung fu fighting, sword master of a princess from a forced marriage to the handsome Dominic West. So there isn't quite as much at stake.

But I don't think that's where the film falls down. "John Carter" isn't a bad story by any means. "Avatar" didn't attract huge crowds because they really wanted to see native people kick corporate assets. It attracted people because of the sheer spectacle. You buy your cinema ticket, knowing you're going to spend the next two hours on a fabulously exotic distant world.

When you leave a cinema at night and the mall and car park are dark and quiet, do you ever felt a twinge of depression? Isn't it a bit like the feeling you have when you get back home after a wonderful holiday? Do you ever notice that this feeling is worse after big spectacle films like "Avatar"? Spectacle in a Hollywood blockbuster should not be underrated.

Most of "John Carter" was filmed in Utah, and it kinda looks like Utah. It's all a bit samey. That's a problem for this type of film, but not necessarily a terminal one. A Western set in Utah could have promised and offered more of a spectacle. Rather, it seems to me (looking at this from the viewpoint of a fantasy writer who builds worlds for a living) is that for any spectacle to be effective, the viewer has to be able to suspend their disbelief – has to allow themselves to be transported there. I've said this many times before. In Fantasy, no matter how weird a new world is, we need to believe, on some level, that it is real. If you're not remotely convinced that what you're seeing or reading is real, then why should you care about the characters?

Pandora and Middle Earth are fictional places, and yet I can believe that they are real. They are separate from our reality. My mind allows them to exist somewhere. Hogwarts School of Wizardry works fine because it's magic, and it's enough that magic could be real. The problem with a fantasy set on Mars is that you can look out of your window at night and see the red planet glimmering up there in the sky. And you've seen the photos and you know that it is airless and dead. And there aren't any four-armed green men or airships. Edgar Rice Burroughs wrote his Barsoom stories at a time when it was conceivable that Mars might be populated. Today is different. Subconsciously, we know before going in to see "John Carter" that it's going to be difficult, if not impossible, to suspend our disbelief, and that ultimately makes the film a waste of time for most of us.

Which is a great shame. Fans of classic science fiction will enjoy "John Carter", even if there aren't enough of us around for Hollywood to make its money back.