The reason is because of the Eviolite.
Dusknoir has a max of 405 for either defense, while Dusclops gets a huge max of 591 for either defense, due to the Eviolite raising Dusclops' defenses by 50%.
This makes Dusclops a very sturdy wall in UU, better than Dusknoir due to it's superior defenses by a long shot. The only roles that Dusknoir plays that Dusclops can't are the role of a bulky attacker or a SubPunch set, which are outclassed by other Pokemon in OU and UU who hit harder or have a better movepool to choose from for coverage along side of Focus Punch.