Goals surrendered:
Jets 103
Sabers 127
Sharks 155
If it's about defense, only one of these D-Men is leading his team to a strong defensive performance. Morrissey also has an existing reputation as a strong defensive player, something the other two lack. In terms of advanced stats Morrissey may be having a weak season by his standards, but a lot of that comes down to a long stretch of games earlier in the season where he was partnered with Pionk, who's always been a train-wreck defensively.
WRT Dahlin's offensive numbers the Sabres score a lot more goals than either the Jets or Sharks. While his totals are similar his points percentage (50.6) is much lower than either Morrissey (61.3) or Karlsson (65.9), so the numbers themselves are a lot less impressive.
I've said before that Karlsson needs to hold a 10-15 point advantage in points at season end to win the Norris and at present he is tracking towards that. Morrissey has separated himself as the clear number 2 choice.