I don't see why people assume because there's inventory left that the game isn't selling. If anything they're holding inventory to try and sell stuff that actually isn't moving (season tickets/plans).
You NAILED it on the head. Islanders-Rangers is the one that's not selling. Rangers-Devils is pretty much sold out. I'm sure the Devils held out on selling them all to push a few more season tickets or small-scale corporate sponsorships. It's pretty well documented that the Devils-Rangers game has been the much bigger draw.