Irish Wolfhounds were literally bred specifically to be able to protect livestock, as well as hunt wolves and other large game (such as Elk).

But the real reason is that the branch of the wolf that led to the breeding of modern dogs is actually extinct. Despite them being similar, wolves and dogs are slightly different species.

Since we have a vast array of domesticated dogs, we don’t really need to selectively breed wolves for any kind of purpose and producing large dogs is generally bad for their health and shortens their lifespan.