Because the Jailer tells Bolvar that the Lich King's role was to pave the way for his entry into Azeroth. So then Ner'zul was just doing everything because the Jailer was "secretly" in the back of the Helm of Dominion making him do it. Its such bad retcon writing dude.
Well, the lore also says that each of the Lich King(s) also resisted him and didn't fully go by his plans. At least that is how I interpreted it. But Arthas and Ner'zhul were definitely influenced by the Jailer in a way.