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Because GB Is "Great Britain" (England, Scotland, Wales)[1] which does not include Northern Ireland.

UK is "United Kingdom of Great Britain and Northern Ireland" which is all four.

It was a politically motivated move basically.

[1] Lets not get pedantic here.

IANA considers .tw to be "Taiwan, Province of China", as does that ISO link. As I understand, China pretty much bullied everyone else into not recognizing Taiwan so the rest of the world (like the UN) agrees Taiwan isn't a country. That's why one reason Microsoft asks for your Region, not country.

There appear to be other ccTLDs like .IO and .AQ that aren't countries. (Probably more.)

And I'm just being pedantic.

Both IO and AQ are ISO country codes as well. I'm not sure why.

Hong Kong has a ccTLD (and an ISO country code, and a currency) but is not a country, while still mostly working like one.