We lost this game because we had the worse goaltending, for god knows what amount of times this season. Our PP was bad, our shootout was bad. Islanders were only 1 for 5 on the PP, we were 0 for 3. They really weren't much better there at all. Hart went 22 for 24, .917. Sorokin went 30 for 32, .938. Their goalie didn't let in a SO goal, ours did.
I'm not exclusively blaming Hart or even attacking him specifically. We can point out the numerous flaws on this team all we want, including bad D and bad special teams and bad coaching. No team is winning when your goalies are posting historically awful numbers on the season, when they're regularly the second best goalie on the ice, and when .917 is the high point of your goaltending performances. That's around average for a good to great NHL starter, it's probably the best we can expect this season at any given time.
Specific to this game, there were several reasons we lost. On the season, goaltending is number one and even with this game, we had the second best goalie by a wide margin. Islanders games typically aren't high scoring games.