This is because Thrawn is much easier to pronounce compared to his full name, which is "Mitth'raw'nuruodo". To that end, "Thrawn" is what's known by the Chiss as Mitth'raw'nuruodo's "core name".

one that gave birth to my book How Star Wars Conquered the Universe, readers filled the comments with his name, usually followed by a number of exclamation points. Thrawn finally showed up on our ...