The book Ender's Game has a psychological component that it's nigh-impossible to nail in a visual medium with child actors. The story works in book form because books are the closest thing we have to telepathy, but it's harder to do in a visual medium simply because visual storytelling is different from written storytelling.

You could probably do the movie with really good adult actors--but most of the cast are children. And really good child actors are rare to come by--you're lucky to have one, much less multiple. And when the cast is made entirely up of children who are all supposed to be geniuses, it's unlikely you'll be able to get the casting and talent you need.

The Ender's Game movie wasn't terrible--it was surprisingly watchable compared to other adaptations of other books--but it didn't come close to nailing the feel of the book.