The image lacks perspective or a vanishing point to give depth. The steps and the torii obviously appear to be cloned
with the angles of the steps, and the thickness of the torii poles. This is where you are getting the
"flatness" referred to by other comments. Think about it for a minute, if you are looking at an object close
up and then looking at the same object in the distance - are they meant to be the same size? That's the issue with
the torii and steps, they are all the same size irrespective of the distance from the viewer. This consequently screams,
with all respect to the effort you've put in so far, a lack of attention to detail. Even if it is a background
(notice I don't say "only"), the viewer's eye will be drawn to the background in a wallpaper as much
as, if not more so, than the character/s themselves.