I love to find old Bruce Eder entries in Allmusic.com:
In fact, the main title theme proved so rousing that it quickly took on a life of its own. Starting in the early ’60s, The Magnificent Seven theme was licensed by the makers of Marlboro cigarettes for use in a series of Western-themed commercials (replacing a much more non-descript working man image previously used in their television ads) that ran for the remainder of the decade and right up until the end of legal cigarette advertising on television. In the end, it may have become the most widely heard piece of movie music in history, allowing for the hundreds of thousands of airings of dozens of commercials for the cigarettes, all of which used at least a fragment of Bernstein’s music.
Ironically, the company that released the movie never capitalized on the music’s popularity, and until 1999, there was no original soundtrack album for The Magnificent Seven. At the time of the film’s release, Bernstein wasn’t well-known for his Western theme music. That soon changed, but not in time for United Artists Records to do much about it. Additionally, United Artists Records was a new operation, only a couple of years old, and had not done particularly well with the Western soundtracks it had released up to that point, some of it very good and attached to even higher profile productions than The Magnificent Seven. By the time the music’s popularity was achieved and recognized a year or so after the release of the movie, the assumption was that it was too late to capitalize on it by belatedly issuing an album, especially since one hadn’t been prepared from the original film recordings.