The composition is so simple - one character, squarely in the center, floating in a starry space. He is poised in a protective stance above the planet he is isolated from, a child with the grim expression of a much older soldier. THERE IS SO MUCH IS PACKED INTO THIS ILLUSTRATION. (You can read about the making of this cover at Tor.)
Also great is the unconventional design of the cover, designed by Jamie Stafford-Hill. Both the title and the author's name are in the center. The neon green text seems to float above the scene, emphasizing the three-dimensional space.
Now look at this poster for the movie version of Ender's Game.
It's not a bad poster - in fact I think the designer did a great job of making all those elements fit together as a cohesive whole. He or she was probably working under a lot of restrictions. But does the poster really say anything unique? Does it have any emotional appeal? The only thing that separates it from any other sci-fi action movie poster is the presence of the two kids in space suits - who are dwarfed by the giant heads of Harrison Ford and Ben Kingsley. It's even orange and blue. There's even a lens flare.
It has more things but it's not better.
Why is the book cover, rather than the movie poster, so powerful? Because the team that worked on it chose simplicity and creativity.