Why would Jet2 want to try STN-JFK or indeed STN to anywhere in the US? The London area is already well covered to the US.
Surely if they were going to try long haul there would be more sense in trying NCL-JFK as there is no US service from there at all at present.
Any airline trying something new would want to minimise the risk they take and an open playing field like NCL to the US suggests either (a) there's no demand or (b) a good chance of turning a profit.