What decides which movies can be played instantly on Netflix?

Why does Netflix offer some movies Instantly and others by DVD? Wouldn't it be easier if they were all Instant with an option of DVD? What really threw me for a loop was Iron Man 2 is on Instant when the first Iron Man is DVD only, even though Iron Man 2 is more recent. If someone could explain the process Netflix uses to determine which movies should be DVD only and which should be Instant/DVD it would be very much appreciated.