Although novels and films are occasionally adapted into plays, it is more often the case that the stage versions are musicals and your question seems geared to musicals, so I'll focus on that.
I think musical theater creators adapt works from other media for several reasons:
Musical theater librettists, and other members of the creative team searching for ideas for musicals, are looking for stories that lend themselves to musical theater treatment. One obvious place to find them is in novels, films and non-musical plays that have the requisite qualities they are looking for (romance, plot, subplot, feeling, etc..... See Lehman, Words and Music, for a bible on what values go into a good musical play).
Often the impetus for a musical starts not with the librettist but with a producer or a composer. Producers or composers are generally not librettists (although they are sometimes lyricists and sometimes, although less frequently, may also be librettists). Those who are not librettists generally look for stories for their shows in literature, in film, and in real life; where else are they going to find inspiration given that they themselves are, as a rule, not novelists, playwrights, poets or screenwriters?
As has already been noted (see AC, above), there is a long and very successful history of musicalizing material from other media. Verdi found inspiration in, among others, Shakespeare and Schiller, for instance. Rodgers and Hammerstein took inspiration from novelists (like Steinbeck and Michener), playwrights (like Molnar) and memoirists (like Leonowens). The list goes on and on...
Given that the great majority of successful musicals (as well as a lion's share of operatic works) were taken from works from other media, that is a tradition that many people working in the musical theater often favor in creating new works.
Even before the age of opera and well before the age of musicals, playwrights often looked to other media and stories for inspiration. Shakespeare generally found inspiration in stories that had already been told, often written in other forms, including the histories of the kings and queens of England, often as had already been written down by Hollinshead in his Chronicles.
And of course, films very often come from novels and plays, including musicals. People who love novels, plays and musicals often clamor for films to be made from works they love (see the countless posts on this site on this topic).
Writers in all manner of media often long to have their works adapted for both creative and financial reasons.
Lastly, people who work in the musical theater often also work in and are almost always at least usually interested in "legit" theater. They are also generally interested in
film (which might also be part of their show biz resume) and literature. When they come across something in these other forms that makes them think, "Gee, this would make a good musical," they often follow through with that idea. It would be odd if they didn't.
Updated On: 8/18/11 at 03:05 PM