Why does not IMO solve the problem of air pollution from sea-going ships by further restricting NOx and SOx emissions?

Although shipping consumes fuels with high levels of nitrogen and sulphur oxides emissions, marine transportation is still considered environmentally friendlier than other modes of transport, since much less energy thereby fueled is consumed in the water ways. It is a fact that ship emissions are lower than truck emissions in terms of ton-kilometre.

The IMO members in general do not feel that ships are the number one problem, on the contrary, a shift to sea transport is evaluated as environmentally friendly.