I would say that Nunu is much like Garen was when they gave him his VU this time last year; Low Risk, High Reward. He doesn't need any work done to make his kit cohesive with his theme, and he doesn't need that much work for most of his skins. The only thing that is stopping him from getting a VU for these reasons is because of his Nunu-Bot skin. Riot would most likely want to upgrade that to some degree (as they do with all champion skins on relaunch/VU's) but since it would be the first legendary skin they would update, it would set precedent on the matter, and that is something that Riot has to think over carefully before moving on it. I think that they aren't going to be giving him a VU at this time because they want to focus some more on other champions.
Also, Nunu isn't very high on the priority list compared to someone like Sion, so there is always that stalling a VU.