Although I am not a Yank Stuk I have had similar discussions with a few over the years and was informed that the main reasons are that it is easy to obtain residency, own freehold real estate and it has a pretty stable democratic govt setup. Not too mention the price, food, wildlife, surf and other fun stuff!! I guess also its the classic case of the sheep following the sheep and because many sheep have been there before good systems are in place for the new sheep which just means more and more sheep keep arriving!
A mate brought a large parcel of land over there in the late 90's for $35k USD. He subdivided it into 2 blocks and sold them 2.5yrs later and made over USD $100k. He was pretty happy with himself at the time but has regretted it ever since. Those blocks might be worth a small fortune now but what annoys him was that he does not own either of them anymore and will probably never be able to again !!! He did use the money to get himself into a nice place in NZ though at the time so its not all tears