I think it is due the magnetosphere shielding from solar wind. Earth has a strong magnetosphere, while neither mars or venus do. For titan, wikipedia says "Tita

I think it is due the magnetosphere shielding from solar wind. Earth has a strong magnetosphere, while neither mars or venus do. For titan, wikipedia says "Titan spends 95% of its time within Saturn's magnetosphere, which may help shield it from the solar wind.", not sure about Triton and Pluto but they are much further from the sun so that probably helps solar wind not strip away as much nitrogen.