This was probable done because the original game only contained the islands of Vvardenfell, while Mournhold, according to the fictional geography of Tamriel, lies on the mainland and a considerable distance inland. The other districts of Mournhold will be absent from that ocean. If a player would managed to get up in the sky or outside the city, they would find that the city was floating in an endless ocean. Because of this the player can not levitate in Mournhold, because that would reveal that the sky is just a ceiling on the interior cell. The creators of the game still managed to capture the feeling of the open-air towns, while this is not the case. It is located in so-called interior cells and thus only accessible through teleportation.
Opposed to the main game, Tribunal, or the city of Mournhold, is not located in the open world. The title refers to the three 'Living Gods', who are referred to as the Tribunal. The main location for this expansion is the temple/city of Mournhold, the capital Morrowind, located in the largest city of Alamalexia. The Elder Scrolls III: Tribunal is the first of two expansions released for Bethesda Softworks' The Elder Scrolls III: Morrowind and was released November 6th 2002.