Why do you think Kovu became the "Chosen One"?

I believe he was born like couple of months before Simba returned and so Scar had time to observe his potential in comparasion to Nuka and Vitani and whoever other cubs might have been there.

He looks a lot like Scar which may have pleased Scar, and may have reminded Scar of himself by also being the younger brother, and also, Kovu may have shown qualities Scar thought good for a king. I think there lies enough reasons for Kovu to get to be the Chosen One.

I agree too, plus he's much better looking than Nuka.
That's why Nuka is jealous because Kovu is the chosen one and not him.