Original: This copy came to Salisbury because Elias of Dereham, who was present at Runnymede in 1215, was given the task of distributing some of the original copies.
Simplified: This copy came to Salisbury because Elias of Dereham was present at the meeting with King John at Runnymede in 1215. He was given the task of distributing some of the original copies.