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".

Among the reveals were Star Wars: The Vintage Collection Sabine Wren & Chopper (C1-10P) 2-pack, Clone Commander Rex (Bracca Mission), The Black Series Grand Admiral Thrawn, Droideka Destroyer ...