IIRC, the copyright on the film itself lapsed since the creators failed to renew it. The copyright on some of the music within the film was renewed, however, and did not lapse. The film producers probably got an agreement with the copyright holders of the music not to license it to anyone else who wanted to use it with the film.
By my understanding, if someone wanted to release a version of "It's a Wonderful Life" with all the copyrighted music replaced by something else, one could legally do so. Such a person would be so thoroughly slammed for altering a classic, though, that I don't think one would likely bother.