I would guess one of the reasons they used 4:3 images was because it fits nicely on both a 4:3 and a 16:9 screen, where as a 16:9 image doesn't do that so well.But it seems that the art and everything else, is specifically made in 4:3. This makes the game worse IMO...
Having to draw both 4:3 and 16:9 based images would basically double the amount of art-work needed, which is one of the advantages of using 3D rendered images because rendering both is generally just a case of changing a setting and then waiting a while for the second image to be created.