Another reason is because Emirates went in at the deep end, dealt with the crappy first few years, saw improvement, continued investment and ultimately led to profitability. They nurtured the route, they gave people what they want (daily, affordable, one stop flights to pretty much anywhere from their local, with quality service down the back - i.e where it counts from NCL) and it paid off, and now they want 2 daily...
United did not do that - and that's why I'd have more faith in Emirates doing JFK. The only problem I can see coming with that is the fact there would be 2 77W's on the ground at the same time