Comment by kijeda

... and .hm and .nf.

The history on why some external territories are coded is because an international standard is the basis for the two-letters used for these TLDs, and that standard is used for other purposes. Far flung localities are often coded due to things like physical mail delivery or customs boundaries which utilize the same standard.

Why does .eu exist? Essentially because "eu" was coded for the European Union to facilitate "EUR" as a currency code when the Euro was introduced. ISO 4217 currency codes derive from ISO 3166 country codes.