Any number of factors could explain why a "Supernatural" movie hasn't happened yet, but the most compelling one is: because it's not done being a TV show.

The TV-to-film transition is rarely done with much success, and almost always when the series has ended. "Supernatural," now in its ninth season, may not even be close to the end.

"With this season -- and this isn't just a line -- we can go for as long as [viewers] and the writers want it to," said series star Jared Padalecki at this summer's Comic-Con event.

Series show-runner (an industry title that means, more or less, head writer) Jeremy Carver agreed. "I have an obligation to plan forward for an arc, but also to keep the show in a position where, if they wanted to go for season 11, 12, 19, it can."

As well, sci-fi/fantasy shows with complicated mythologies rarely make successful movies.

The two recent examples that fit best are "Firefly," which spawned the film "Serenity" (2005) after its cancelation, and "The X-Files," which produced two films, one in 1998 and another in 2008. None of these films was a hit. ("Serenity" may have made its money back on video later, but that's the sort of gamble studios are generally unwilling to take.)

That said, one can always cite the original "Star Trek," which wasn't particularly successful in its first run on TV in the '60s but was turned into a hugely successful film franchise years later.

But that's one success against a number of failures, which is again the kind of math studios don't like.

There's also the fact that "Supernatural's" creator, Eric Kripke, is busy with another series, "Revolution," and is thus unlikely to have spare time for a film.

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