It was always an American company. In order to launch rockets from countries in the US sphere of influence (even from NZ), companies must obtain an FAA license.

Rocket technology itself is so intensely regulated by US export control laws that it’s practically impossible to develop an orbital launch vehicle without being a US- or Europe-registered company.

It is a real shame. It also looks like a lot of engineering work is shifting away from NZ — Auckland seems to be focusing more on operations and space systems, and the launch stuff is moving to the US with Neutron.

https://en.wikipedia.org/wiki/Rocket_Lab#United_States_move_...

SpaceX previously said that are not allowed to hire foreign nationals generally.

So guess NASA told Rocket that if they want American contracts, they need to move?

https://qz.com/794101/elon-musk-explains-why-he-doesnt-hire-...

Needs access to American capital markets, contracts, governance structures, and jurisdiction (applicable law).

same thing that always happens to companies, money

Capital probably, market access. It is pretty hard to raise capital for high risk ventures like that everywhere in the world other than the US.