Career opportunities mostly. The startup I was working in was acquired by a US tech company and they decided to close the NZ office and give us the option to relocate to San Francisco (or stay in NZ and find another job :). I was pretty happy with this situation though as there isn't much in the way of a tech industry in NZ, so there's much more interesting work available here, especially in the bay area (same goes for my wife who is in biotech). Oh and despite the insane living costs in the bay area, the pay is far higher than you can get in NZ, I make more than three times as much here as in NZ (Plus NZ tech companies are notorious for not giving out equity or stock options to employees - even early startups).
NZ has its good points with regards to being able to have a more relaxed lifestyle, and its great for outdoorsy types - but skiing, hiking etc. doesn't really interest me that much, and I kind of like the bluster and hustle of the bay area.
As a New Zealander going through the US immigration process, this whole post rings extremely true to me. People who have not gone through immigration before have no idea what its like for your entire life to be dictated by the whims of an opaque bureaucratic system. The most frustrating thing about it is that you have essentially no opportunities to better your situation via your own initiative. Don't like your job, well you can't go and find a new one unless they are willing to sponsor you for a visa. Want to start a side business - nope, thats a grey area that I've had immigration laywers advise me against. Think you deserve a promotion - nope, your residency application was for a particular job description. The list goes on.
A few months ago, I came very close to being deported due to the business unit I worked in being divested to another company - this isn't a problem for H1B employees, but I had an L1 which is non-transferrable to a new company, and since my job at my existing company was no longer present I would be made redundant if I couldn't get employment at the aquiring company. Luckily for me, my residency application became current 2 weeks before the aquisition closed (I had been waiting on this application for almost 3 years), so I was able to move into another immigration status that allowed me to move over to the acquiring company.
If that hadn't happened I would have been left with no job, A pregnant wife (who would also lose her job - as her work auth is tied to my visa) and facing an expensive move back to NZ (relocating a family internationally is really expensive btw). Fortunately for me I'm now on the home stretch (I hope) and I check my mailbox religiously waiting for the day my greencard turns up, and I can start living life on my own terms.
NZ has its good points with regards to being able to have a more relaxed lifestyle, and its great for outdoorsy types - but skiing, hiking etc. doesn't really interest me that much, and I kind of like the bluster and hustle of the bay area.