How does iTunes pick which movies will go on sale?

They really don't have all movies, they usually add movies that were really popular.

It depends on which movies they can secure the rights to. In order to be able to sell the movies the studios have to agree to let them, just as artists have to with their music. Some studios and rights holders, don't want to share the wealth, so they are not on iTunes, but new deals are struck every day.

Huskie#9 said:

Do they just pick random ones or do they pick specific ones? Because, I was kind of waiting for the American Pie films to go on sale. Please help me.

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You can check out this service KinoHunt that allows you to track movies you want to watch and informs you when they go on sale.
They also have an iOS app here making it convenient to stay on top of movie deals: http://bit.ly/1tmSctF