Why did Turkey become Türkiye? I think mostly because they asked. I’m guessing that Japan/Nippon is enjoying the fact that English speakers use the Chinese name for Japan and the Sanskrit¹ name for China. It’s much like the Czech Republic became Czechia, although part of that was Czech speakers wanting to stop referring to their country as an adjective² (the Czech phrase for Czech Republic was often shortened to just Czech).

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1. As a kid, my dad had told me that China was the Japanese name for the country, but according to Wikipedia, the name is actually derived from Sanskrit.

2. Which reminds me of the fun challenge of Czech (and many other Slavic languages) is that unlike other Indo-European languages³, the declensions of adjectives follow a different pattern than the declensions of their corresponding nouns,

3. Or at least the Indo-European languages that I have familiarity with.

Turks did not really want it to become Türkiye in English, it was a government push. Most of us prefer having the name of our country be pronounceable and writable by anyone talking about it, and no one will even notice if you call it Turkey.

People just liked it better that way.

The answer is as simple as “they asked nicely”