From a mechanical perspective, it's because he's already used his reaction on Round 2 to cast Chill Touch, and you only get one reaction per round.I must say I'm intrigued. Trying to wrap my brain around...Why can't Vlad use his reaction to cast Shield? Is it because he was readying an action? The round before he just readied an action to cast Chill Touch, I didn't see him using a reaction.
From a fictional perspective, he's still finishing up casting Chill Touch when the arrow thunks into the wagon right below his ribs. He can't cast Chill Touch and Shield simultaneously.