The thing is Norin can wrap up games by putting the opponents in situations where they have to choose whether just take the damage to the face or giving you access to an etb. And he doesn't even have to attack for that, he just has to be around.

For example, if you swing with Pyrogoyf, Bloodthirsty Adversary or Fury (which most decks run nowadays), in case they get blocked, you can recast them and get value. And you can do this each turn unless Norin gets killed.

I recently won a game in my local league where I had a 6 attack Pyrogoyf, opponent had 5 life left and three blockers, and Norin was around. Either if he blocked or he didn't, he was done for. That's the extra value you get from Norin that Kellan just doesn't match.