Smoke, remember that elk have a much, much thicker hide than deer. A bullet that can pass through a deer and go on to the next county might not bust through an elk's shoulder 
Less of an issue if you don't shoot an elk through the shoulders. Seems there's a large area back of the scapula that doesn't require a super bullet to get the job done.
While this is true, sometimes stuff happens and having something a little more advanced than a round ball can make a big difference
