Usage, Placement in OU is decided by how often the pokemon is used. So since Dusknoir is the slightly stronger option, he's the one used in OU, and Dusclops oddly enough is almost never used in UU.
I feel that Dusknoir deserves to be tested in UU. Would you like to help me test Dusknoir, Tentacruel, Electivire, and/or Mamoswine in UU? Any 1 of which or any combination would be helpful. Ideally, you could submit the footage to you battle recorder and upload it to Nintendo's servers, and write down the battle's ID number so others can review it.