Because he wants to gloat.
This is a trope known as "Evil Gloating"
Gloating. Monologuing. It's what villains do. Maybe they "set up us the bomb" and are calling to rub it in and take credit, perhaps he's just snatched the MacGuffin from the hero's very hands and insist on staying just out of reach to taunt. Maybe the villain finally has the hero at his mercy and just has to taunt him one last time before shooting him.
Or perhaps this is the place where they finally have a chance to get all the reasons they despise the hero personally off their chest. Any mistakes the hero makes are also fair game for villainous gloating. At any rate, while this is a villainous staple on par with a hero and In the Name of the Moon, it's still one of the things the Evil Overlord List strenuously warns against, as any villain caught gloating is sure to be either killed or foiled, as in the end his gloating always ends up giving the hero that little bit of extra time or information that helps him counter the villain's Evil Plan.
Of course, in this case Viggo would have killed Wick who only escapes due to outside help. So he was justified in gloating this time.