Ariza is simply a better defender than Artest right now. Artest was always a pretty selfish defender - he'd shut-down his man but would help a little as possible - and nowadays he can't defend the quicker perimeter players anymore. Still great to defend longer, slower power 3s, especially if the refs are allowing him to be physical, but Ariza is more versatile, can guard every backcourt position equally well and a better help defender.
Maybe Artest is still better offensively, in spite of his atrocious shot-selection, but as stated in the article, he's the 4th/5th option, so who cares? Ariza is a better 3pt shooter now, he'd be a better fit for the Lakers on both sides of the floor.