How come some movies on instant netflix disappear after a couple of months?

I remember there used to be a lot of movies available instantly on netflix but now they are no longer available. For example, I wanted to watch the proposal, every time I would go on the netflix instant site, it would be there I never really wanted to watch it until last night when I searched it, it's only available on DVD not on instant. I distinctly remember that movie was available was on instant. Is there some time limit for movies on netflix to be shelved on instant?