why play nunu when you have ivern?

cause Nunu is sitting on a 57% wr in Diamond+ and 54% in general atm.

because you have a Cait on your team

They are for different things. Ivern is more of a support than Nunu is while Nunu can easily secure Dragon and Baron by himself. He has smite on a 5 second cooldown as well as normal smite on top of that.

Nunu is good at counter jungling (yea better than ivern, ivern can't heal and he can only take 1 camp at a time with smite, and he can't wait in your jungle for the rest of them), and nunu is really good at objective control since, unlike ivern, nunu can actually take dragons and barons easily on his own.

Ivern is more of a supportive champion who helps his team by basically being jungle janna and giving everybody buffs while rushing redemption. This is why ivern isn't a flashy kind of broken since he wins games by giving teammates such an advantage and peeling for them. He's a support, nunu is a tank/counter jungler, and if you're playing ivern strictly to counter jungle well that's not being too useful and that's ivern's entire playstyle, being useful.

Ironically nunu counters ivern just because his counter jungling is far superior and ivern can't really do anything about it due to how tanky he is and that he can even eat and heal off daisy.