Why have efforts to reduce sulfur oxide emissions met with greater success than those directed at nitrogen oxide emissions reductions?
There may be multiple reasons, but one factor is that nitrogen oxides can be made from the air itself.
To better understand what makes nitrogen oxides different, let us start with sulfur oxides.
Our atmosphere does not naturally contain a significant amount of sulfur-containing species. We can get sulfurous compounds from volcanoes but they soon react and end up as condensed, nonvolatile materials like sulfates. So, the only way our burning of fuels can generate sulfur oxides is if the fuel itself is contaminated with sulfur. We can clean out ("scrub") this sulfur contamination before burning and we're all good.
Eliminating nitrogen oxides is not so easy because nitrogen is in the air, and the heat of combustion processes can make the nitrogen react with oxygen and make nitrogen oxides. Even supposedly the cleanest possible fuel, hydrogen being burned to make water, could give nitrogen oxides when we burn it in nitrogen-containing air. We are stuck because we can't scrub nitrogen out of the air the way we scrub sulfur out of fuels.
Moreover, we face a dilemma because high flame temperatures, which we want to make the combustion more efficient, also cause more reaction between nitrogen and oxygen.
As long as we depend on burning fuels for energy, even if it's hydrogen generated via solar energy, special combustion technologies -- or improvements in process efficiency that enable burning less fuel --
are needed to get around these problems and curb nitrogen oxide emissions.