Mostly because of what Fated Fathom said:
>That's because most people don't run Dusclops at all. And since it's attacking stats are no good, Night Shade is preferred for consistent damage.
commented 42 minutes ago by Fated Fathom ♠
Dusclops is bulky af already. Eviolite is even better. But nobody runs it because Taunt shuts it down completely.
If you mean in the game, same reasons + Aegislash. This thing is all over the ORAS metagame. And running one myself, I know this thing is lethal af and can sweep quite easily if given the right opportunity to set up.
Hope I helped!
-It's a secret!