There is no exact reason they gave Ho-oh Burn Up in USUM rather than SM. Many other Pokémon got new moves in the game that they didn't have before. However, most of these moves are the ones you exchange BP for. For example, Tapu Koko can learn Thunder Punch in USUM, but couldn't in SM.
The reason it is Ultra Wormhole exclusive, obviously, is because the only way to get Ho-oh in the game is by going into an Ultra Wormhole.