Two letters TLD like .io are ISO country codes. Catalonia's .cat is a generic TLD in comparison. Since .io stands for the British Indian Ocean Territory and Chagos Island isn't going to be 'separate' anymore by becoming part of Mauritius, IANA's logic is that the ccTLD has to be retired. That .su is still around after the collapse of USSR isn't a valid argument for them.

Technically, they haven't yet. In the past they've sometimes transitioned ownership of country-code TLDs over (like the Soviet Union's .su to Russia). I think they just don't want to wade into the debate over the Chagos Islands in general.