Why do songs sometimes get left off a movie's soundtrack?

Question

Sometimes I'll hear a song while watching a movie, and then later when I look at the movie's soundtrack, I find out the song isn't on the soundtrack. Why are songs in a movie sometimes left off the movie's soundtrack?

As an example, the movie American Wedding had a scene with Stifler in a dance-off. During that scene, they played Michael Sembello's "Maniac", Belinda Carlisle's "Heaven Is a Place on Earth", Eurythmics' "Sweet Dreams (Are Made of This)", Bananarama's "Venus", and Duran Duran's "The Reflex". That was arguably the best scene of the movie, and yet none of those songs appear on the movie's soundtrack. The soundtrack was only 60 minutes in length, and since a CD can fit about 80 minutes of music, they could have squeezed most or all of those songs onto the soundtrack.

Answer 1

Licensing mostly. Artists often belong to different record labels, and their rights have to be negotiated. Labels could prefer to not the licence a song, in an attempt to sell their own material; single, album, EP, or compilation. Looking in an album leaflet, you might see artist x appears courtesy of x record label for certain licensed tracks.