Why does New Zealand seem so encouraging of immigration?

I was reading a book about immigration that covers immigration into many individual countries. It said New Zealand was has launched it's own marketing campaign to attract middle class American immigrants. What are the reasons? I checked out their website, and I must say, I've need seen a country provide such a helpful and encouraging web page devoted to immigration.

http://www.emigratenz.org/

I checked out their various lists of desired fields of employment for new immigrants, and the list seemed rather varying and extensive. Why the sudden need for so many different types of professionals?
Additional Details
Part of the reason I'm confused is that my father (who worked abroad in many countries as a professor) said it used to be a difficult country to immigrate to. If he was correct, I'm wondering why this has changed, and what developments lead to this change.