Now when Scar is King there's hardly any food, so maybe the pridelands and Chumvi's pride are close enough together that they had no food as well. Maybe Chumvi's pride was starving to death, so Tama, still trusting Zira takes her cubs to her so she can raise them. Since there were no other cubs than Kovu, Vitani, and nuka, scar chose Kovu to be the next king because Vitani is female and nuka is just...Nuka. Therefore Kovu was the strongest cub.
Other possible parents