Answer:
The principal difference is that in a movie, the viewer does not have any impact on it, the viewer is just that, a viewer. On the other hand, in a game, the player has an impact on history.
So in a movie, the scenes are well defined, so the composer has exact rules for working, while in a game, you do not really know how many time the player will spend in some area, so the music in that area can not have a defined length, must be looped.
There are other differences, for example in a movie, the composer has something to work, some scenes, some imagery, etc.
In games this may not be the case, because you can not send scenes or "pieces" of the game to the composer to work with, so he actually has fewer things to work with.
