The Redskins used to have camp at Dickinson College in Carlisle PA so it made sense to scrimmage because it was just a short bus trip. Then the Redskins built that new stadium and a new practice facility nearby where they started charging to attend.
Most teams used to have camp away at a college but when the new wave of stadiums got built, most teams also built a practice facility and chose to stay home. The Steelers built one too but they still choose to go away for camp.
Of course, teams used to be able to hit in practice too so scrimmages made sense.