Doesn't this same vagueness apply to the first as well, why does scar hate Mufasa so much, why are the hyenas exiled to the elephant graveyard. There's some kind of history there but we don't get it nor do we get anything on how any character but Simba developed. Were the hyenas Scar's actual friends at a time is that why they took his betrayal so hard or were they just sick of his arrogance and false lies. Did the hyenas kill Scar because he was a genuine friend who betrayed them or because he didn't live up to his end of the bargain. Either one of those completely changes the meaning of his death scene.
The first movie was very vague, Nuka as underdeveloped as he was is still far more fleshed out than anyone but Simba and maybe Scar in the first movie. This is a double standard simba's pride doesn't give us answers entirely but there's quite there actually. The first movie doesn't explain anything either we know nothing of the why of almost anything and many relationships are very vague in what we know. While having few direct answers we know a lot more there's no reason to think Nuka isn't Zira's son, if he's scar maybe but it's not important nuka's concern isn't Scar's legacy it's getting his mother's approval.
Who his father is doesn't matter anymore than who Nala's father is (which I might add is another thing from the first movie we don't know) his father isn't important and clearly doesn't matter to him, he wants to prove himself to his mother and gain her love and approval. While there could be more there doesn't need to be his motive is very clear his heritage doesn't matter that's not what is driving him, so him being Scar's son is irrelevant to the matter at hand, Kovu stole his mother's love he wants it back it back.
Note I am not dissing the original no way, but it's unfair to dock Simba's Pride for not explaining a lot when the first movie did the same.