Because Final Mix is superior in content, but the original North American PS2 release (plain Kingdom Hearts ) lacks all the extras. Fans wanted the full experience in a language they could understand.
With the release of Kingdom Hearts HD 1.5 ReMIX (and later Kingdom Hearts - The Story So Far and the All-in-One Package ), Kingdom Hearts Final Mix is officially available in English worldwide on modern consoles (PS4, Xbox One, Switch, PC).
