In the original text of the crucifixion of Jesus, there is not a lot of emotion. In the original text it is simply the facts of the story, there is not a lot of detail used to describe the crucifixion or how those closest to Jesus felt. In the St. John Passion, emotion is added. In this musical piece, the lyrics express the feelings of the disciples and go into detail of the feelings of Jesus and those who are close to them.
Also, the original text is a pretty fast and short story. If the Passion were to just be the original text, even if there were repeats and it was a slow tempo, it would not last very long. The Passion slows down the story because it goes into a lot of detail and focuses a lot on the suffering of everyone. The Passion lasts for a long time because of this added detail to the text.