Playing in away conditions has put a lot of pressure on them in my opinion and they are not being able to get used to the conditions. It has taken a big toll on their performance. They are playing really bad in test cricket right now. They are even losing the matches that they should easily be winning.
New Zealand players are more suited to the longer format, when compared to limited overs cricket. It is their strength and I don't think that they will ignore this format. Both their batsmen (Kane Williamson, Tom Blundell.etc) and the bowlers (Kyle Jamieson, Neil Wagner.etc) are tailor made for test format. Only a few such as Trent Boult and Devon Conway have proved to be consistent performers in limited overs cricket. The excuse of not getting used to the conditions is flimsy. The team is jampacked with experienced players, and weather and ground conditions in England are similar to those back home in New Zealand.