I would say because it's the easiest and most natural way to read/view anything of this nature if english is your primary language (read left to right, top to bottom).
The list won't wrap, so you have to maintain your game list vertically...that leaves left or right side of the screen. If you center it, you won't have room for snap shots.
Just my $.02 on the same layout issue.

Come to think of it though...I may switch the game information and the snapshot images so that more of the ring is visable behind the transparent game info...the snaps won't hide so much of the background if I swap them...
Thanks for making me think
