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.
Things are getting better wrt companies granting options here. The law has improved slightly but still isn't perfect. At least it's simpler than the US.
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.