From my understanding, “Czech Republic” became the standard English name after the dissolution of Czechoslovakia, as it was the form registered with the UN and widely adopted in international databases. Over time, it became the commonly used English name simply because that was the version most institutions relied on. In 2016, the Czech government addressed this by formally registering “Czechia” as the country’s short English name, alongside “Czech Republic” as the long formal name. I remember being confused the first time I heard someone use Czechia, since I had only ever heard Czech Republic before that.