This topic has been beaten to death around here but clearly NZ targeting JFK isn't about connections at that end, it's because JFK is seen as the premium airport at the NYC metro area and they want to target those passengers bound for NZ, as well as those ex-Australia heading to NYC.

There's far better options to Europe and NZ knows that (hello SQ), if you have to transit the US the I-I connection experience at IAH beats anything else in the network without the weather worries (for most of the year, at least)