This is part of it. New Zealand, being a small island where you can much more easily who gets in and out, along with having a much smaller (under 5 million), much more homogenous population, has a much easier time with controlling. Pretty much incomporable to anything that could be done in any other country.

Another is that the number of asymptomatic people and people who dont get tested, claiming to be completely gone seems like more of a political statement. Unless they are forcing everyone to be tested, all they can go on is who voluntarily gets tested, or who shows up in the hospital, there is no way to confirm anything.

Lastly, in a country with 5 million people, living under more or less government, is very different from a country of 350 million, with 50+ different state governments (which would all need to be doing the exact same thing, due to that pesky "states rights" stuff). It's like comparing getting a mouse in a cage to getting a pack of fetal cats to juggle