By comparison, there are 6 million Americans, or ~2% of Americans living overseas. http://en.wikipedia.org/wiki/Emigration_from_the_United_Stat...
If NZ is as great as the author thinks it is he might want to ponder why so many people have left.
When you grow up in New Zealand, it's almost a rite of passage to do your "OE" - overseas experience. It's part of the culture to go and explore the world. And of course young 20 somethings travelling meet other foreign young 20 somethings and it's pretty easy to see where that stat comes from. "Your country or mine? (the girl usually wins)"
I grew up in a small town, lived in Auckland, now live in Toronto, Canada. The Greater Toronto area is the same population as all of New Zealand. Is it any better or worse ? No. It's just different. It all depends on what floats your boat. Are you an adventure sports junkie ? New Zealand is pretty much heaven on earth (I'm guessing Colorado would be awesome if you dug winter and summer sports together). Do you like bustling metropolis ? Well Toronto kinda rocks - Canadian and Kiwi philosophies are very close (although BC is far closer in life style to New Zealand, than over in Ontario)
There is so many factors for what's good in a country - a lot are dependent on the invididual. What will be one persons heaven could be another persons "meh", quality of living, friendships, relationships, career choices, sports, food quality, environment yadda yadda.
As a New Zealander living overseas, in my case, the pull to go home is very strong and ever present. And I will say this when I finally move home, I will never, ever complain that it's freezing in winter.
It's easy to find out why, even in the context of the article. You see what the rest of the world has, and want to take part. Want to work on building a Boeing 777? You can't do it in New Zealand. Sure, you can work at small companies building small planes, but if you want to be working "world scale", you have to be where all the people are.
To relate back to HN, there are a few small startups, but the market is different in New Zealand. As a nation of small businesses, about 95% of the population work for a company with 50 or fewer staff. If your business involves selling to other businesses at scale, there are comparatively far fewer opportunities to do this.
NZ would be a great place to put a remote team (Mozilla have one in Auckland) and Kiwis love the idea a company will pay to fly them around the world every now and then. Even better, if you pay them in USD or GBP, it generally works out favourably for both sides.
It's not quite as simplistic as you make it seem.
If someone wants to leave the US and move elsewhere, they need to be a skilled worker to make it happen. Poor Americans tend to be a net drain on society, so other countries don't usually want them.
But if you are a skilled worker, the economics make it a lot better to live in the US. I recently considered moving to London, but the economics just made it ridiculous [1]. I would have taken a 30-40% nominal pay cut relative to the US (i.e., convert my salary from GBP to USD, it's 30-40% less). The UK was also far more expensive when I visited - compare a 1-2 zone Oyster card (112GBP unlimited monthly) to an unlimited metrocard (104 USD), or the price of brunch (10GBP vs 10USD).
[1] I would have been willing to take a moderate pay cut just to live somewhere different, but the cost of living in London was just too high.
Edit: It sounded like I was disagreeing with your figures, I wasn't.