It should have been obvious that there weren't going to be any new Alolan forms in USM considering Z-moves are the main attraction of Gen 7. Alola already has many exciting new changes since we first visited it in Pokemon Sun and Moon, such as brand new Z-Moves, the mysteries surrounding Necrozma, new Ultra Beasts and an overall larger variety of Pokemon to be found. It is for this reason that new Alolan forms weren't included, as they would be overshadowed by how much everything else in Alola has changed.