Why are only a few corporate businesses benefiting from a resource that belongs to all New Zealanders?

Because 78% of quota shares are now owned by 10 entities.
The resource rentals that initially applied to commercial catches were abolished in 1994. Those rentals were a tax that provided a return to the Government in recognition that a national, natural resource was being exploited for private profits. Quota holders now pay levies, which only covers part of the costs of administering, managing and monitoring commercial fishing.
Many of the small-scale fishing operators do not own quota and lease catching rights from quota owners. Generally, this obligates the fisher to supply their catch to a particular outlet which pays them an arbitrary price for their fish. This denies these operators the opportunity to maximise their returns by selling their catch to their local community or another retail outlet.