I suspect that they simply don't want to pay his rate, so they go with new faces that can barely draw.
As likely an explanation as any.
I was looking at an old Ordway-drawn Superman comic the other day. It had detailed backgrounds -- city buildings, cars, trees, people in recognizable outfits. Ordway (and the inker) obviously put some effort into it.
Most of today's Superman comics have a monochromatic background. Superman will be backed by a wall of light blue. Nobody could be bothered to draw anything behind him.