Another word: Culture.

4.79 million New Zealanders compared with 328 million Americans.

New Zealand has never had the frontier culture that Americans mythologize. New Zealand is a far more collectivist culture (as is the case with the Commonwealth nations as a whole) and they tend to prioritize "What's best for the nation?" over "How does this affect me?"

Another factor: there have never been that many guns in the British Commonwealth nations to begin with. Firearms are seen as a sporting tradition, not as self-defense and not as the means of resisting a potentially hostile or overbearing government (however unrealistic the second case might be in this day and age).

Also, there's this thing called "The Second Amendment". Change the Constitution and then the confiscation can begin. Good luck there.

You won't believe it from this post, but I'm a lifelong liberal and Democrat. But don't mess with the right of law-abiding gun owners. Instead, enforce the laws and keep guns out of criminal hands. Tighten up background checks and close the loopholes. Don't make gun owners who have obeyed the law all their lives into criminals overnight.