The reason it would be hard to do right, which I read on a forum a long time ago and completely agree with, is the backstory that would be required. When someone goes to a 2 and a half hour movie, they expect to be introduced and immersed in the story and see it through to the end. Look at Warhammer - you could fill a small library with the backstory of the Space Marines alone. A movie about them would need to be on the scale of Lord of the Rings, and people just wouldn't go to a movie that big when it's not rooted in a well known series.