They started midnight showings of Rocky Horror because it had generated a cult following that enjoyed watching the movie while acting it out.
Well, Rocky Horror is about outcasts. What Will conveniently glosses over, though, is that the outcast culture that Rocky Horror appeals to and was championed by was specifically queer culture. The midnight showings actually preceded the audience participation aspects of the movie, and the movie was deliberately introduced to queer markets in an attempt to gain traction:
The management of the Waverly Theater on Sixth Avenue in Greenwich Village had begun to run midnight performances of grungy B-movies, cheap to rent from the studios and calculated to lure the pink dollars out of the pockets of the nocturnal weekend denizens of Greenwich Village, Soho and Chelsea… Like the London punks who embraced the first London stage production, the gay audiences have always been eager to find somewhere else to discover and populate for themselves. They soon gravitated to yet another cruisy cinema… (Rocky Horror: From Concept To Cult by Scott Michaels and David Evans)
For all that it’s been adopted by any number of outcast cultures and appeals to any number of outcast themes, it’s still primarily a queer space, and for all that it’s a dying culture, the few theaters that still show it still have a primarily teenage audience, largely because Rocky Horror provides a liminal, carnival, rite-of-passage space where kids are free to play with gender and sexuality in a safe and accepting environment.
I just have a lot of feelings about Rocky Horror as a queer space and queer cultural artifact and dying queer subculture, okay.