Because unlike other TLDs, the two-letter root domains are well-defined as ccTLDs way back in 1990 defining it as "following ISO 3166" two-letter code, and ISO

Because unlike other TLDs, the two-letter root domains are well-defined as ccTLDs way back in 1990 defining it as "following ISO 3166" two-letter code, and ISO has assigned Niue as NU. The only exception now is .UK and .SU (United Kingdom is technically GB but before ICANN it was assigned both UK and GB and Soviet Union is gone but it has became de facto ccTLD for CIS).