Level / boss status doesn't really relate to strength within the lore.

Dar'khan Drathir, the leader of the Scourge in Quel'thalas, was a major villain in the Sunwell Trilogy, a nearly immortal half-lich that was able to hold his own against Sylvanas, Halduron, Lor'themar, Jorad, Kalec, Tyri, and Krasus... simultaneously. And yet he's fodder for level 20 blood elves questing through the Ghostlands (and doesn't look nearly as cool in-game as he does in the books).

I think they simply wanted people to be able to fight Anub'arak. His encounter is, in my opinion, a really good representation of him, and his instance is really short and cool. He might not have been a level ?? raid boss, but I think the instance does him justice.

And, while I wouldn't rule out the possibility that he could return, I don't think it's likely that he'll be re-raised by the Lich King, as his dying words are something to the effect of, "Finally, I'm free of him..."

@Suzaku

I have no issue with the Azjol'Nerub instance (it is one of my favourites) and I accept that there are other occurances of powerful (in lore at least) characters appearing in lower level areas. Gnomeragen springs to mind.

The thing I just found strange was the appearance of a major WarCraft 3 character appearing as a boss in a 5-man when all the equivalent characters appear as raid bosses. I would argue that Anub'Arak was at least on par with Lady Vashj in terms of importance.

I conceed that Blizzard wanted to make an encounter that everyone could experience featuring one of the WarCraft 3 heroes but would have thought it would at least be a level 80 instance.