Why are some prophecies written in the past tense?

Answer:

One way God emphasizes the certainty of what He declares about the future is to express it as if it has already occurred. Scholars call this the "prophetic perfect tense."

Speaking of the future destruction of Tyre, God spoke of it as already being destroyed:

"By the multitude of your iniquities, in the unrighteousness of your trade you profaned your sanctuaries. Therefore I have brought fire from the midst of you; It has consumed you, and I have turned you to ashes on the earth in the eyes of all who see you. All who know you among the peoples are appalled at you; you have become terrified and you will cease to be forever" (Ezekiel 28:18-19).

What would happen to the Messiah was a certainty, which is why Isaiah 53 is in the past tense.