This is such a shame if true. Pity they couldn't have tried a longer season to thoroughly test demand.
It would be easy to blame a number of factors but the marked drop in the pound vs the dollar recently after Brexit must have two main effects:
1. Brits going to the States will find it much more expensive to do so, possibly leading to people planning holidays elsewhere.
2. People paying GBP to buy seats from an airline that does its accounting in USD will mean yields get hammered even if they were good to start with.
I don't know but at a guess I would say the majority of pax on NCL-EWR would fall into both categories.
All sad.