Well, since the code is left-to-right, and the editor is presumably as wide as the longest line, the content will tend to be skewed to the left. By putting the editor pane on the right, you reduce the average distance between the code and the output.
I suppose, though I'd rather it be resizable and let me take full advantage of a monitor. I've been experimenting with this sort of live code preview with http://padhacker.net/ (target platform primarily tablets) and resizing has worked out pretty well. (When it does work. Shrinking the iframe gets sticky.)
