Not sure if someone already made this comment or not, but I really think the issue with NYC/Quebec departures is the time of year they are offered.
To me, the appeal of a cruise with a NYC departure is that it would be more convenient to get to than a southern cruise port.
I imagine a lot of families in the northeast states as well as Ontario, Quebec and the eastern Canadian provinces would think the same.
Problem is, for families in those areas, fall is not necessarily the best time to cruise.
September is "back to school" month for most schools in eastern Canada and the US.
Cooler, even wintry, weather can start to creep in as early as October, which sort of puts a dampen on the whole "let's go on a cruise close to home" aspect of things.
If Disney was willing to offer summer NYC or Quebec departures, I would be willing to bet they would sell MUCH better.
Also, for me personally, I would prefer a few additional Canadian stops on the cruise, even at the expense of a sea day or two.
(If Disney offered an itinerary similar to the ones HAL offers between New York (or Boston) / Montreal in the summer, it would totally be a "take my money" moment.)