To get ashbringer in vanilla was stated to be info by a person on outland… it would be on par with us defeating Illidan in Black temple then for the next patch a npc helps us construct ashbringer or ashbringer variants that we farm up on 4 classic zones being attacked by undead. The final raid is called siege of northrend… starts on shores of Northrend and ends on top of ICC to fight Arthas
So what not every bad guy gets a whole xpac or two dedicated just to chasing them down to kill them. Many of them get a single patch and raid. Get over it.
The difference between this and previous instances is the context behind N’zoth. N’zoth was built up as far back as Cataclysm, as a member of the Old Gods, ancient beings who (at that point) canonically cannot be killed without huge negative consequences (as evidenced one expansion later with Y’shaarj). And on top of that, N’zoth was built up to be the greatest-scope villain of the bunch, being the god behind Deathwing and the one who created the Naga.
How he was handled in his actual appearance was to suddenly break out of his prison in the expansion mid-patch, and be quickly tossed aside right after, when, with all the build-up from previous expansions, he should’ve been free’d right at the start, with the story involving figuring out how to re-seal or otherwise deal with the apparent strongest of the known Old Gods.