Fandom

With the world in the throes of Harry Potter mania, what with the release of Order of the Phoenix today and the next book coming out in just ten days, I was thinking last night about Fandom and how it develops. Fandom, for those who might not have heard the word, refers to the following a particular story/book/universe achieves, and all that that implies--how ravenously fans devour books, how highly anticipated new releases are, the prevalence of fan fiction and fan sites on the Internet.

And it occurred to me that it seems that only sci-fi and fantasy works achieve the mega-followings in the world of entertainment. Look at the devotion of fans to Lord of the Rings, Star Wars, Harry Potter, and the mother of all Fandom, Star Trek. The only non-sci-fi/fantasy thing I can think of that might even come close to paralleling the devotion with which fans seek out anything and everything to do with these fictional worlds is Pirates of the Caribbean, which could probably fall into the fantasy camp but I think of more as action/adventure.

Why is this? Is it a product of exposure (the fact that Star Wars, for example, has probably nearly 100 books published, plus resource books, video and board games, toy lines, etc.). Or is that exposure the result of the fan base?

Any thoughts?

(None of this is to suggest that books and movies outside the sci-fi and fantasy genres don't have their own rabid followings. There is just something about these large fandoms that seems unmatched anywhere else.)