That's not a rhetorical question at all. I truly don't get what makes a champion "high priority" when it comes to reworks.
If a champ is any one of, or a combination of: frustrating to play against (uninteractive and/or lack of counterplay), too low skill ceiling, too high skill floor, too large a gap between high Elo and low Elo performances, too binary (a.k.a. Feat-or-Famine), or poor execution on their thematics. These are reasons that they are high priority.
Nunu kits is basic, sure, but it doesn't really suffer of any of the above issues, or at least none at a severe degree. Sure, his skill ceiling is a bit low, but he still sees pro play once in a while. Sure, his thematics execution is not perfect, and the duality between Nunu and Willump isn't very visible, but he still has character and personality.
I've heard people say Urgot is high priority right now- but didn't he just get reworked?
That wasn't a rework, that was an opportunistic band-aid. If you cut your hand off, it's not because you just stopped the bleeding that you shouldn't go to the hospital. (Also, technically, Nunu received a very similar change in scope when they reworked his passive.)
When is Nunu going to get some love?
Without any unforeseen event that would bump him up the list of priorities, maybe 2-3 years for a full rework. Keep in mind though: he's not in a terrible spot currently, contrary to what you claim, and he'll still be touched with balance changes if need be.