Because it is the world's first photograph ca 1500. Read "Turin Shroud" by Picknett and Clive.
The image is "hinged", i.e. no top of the head shown so it's no wrap-around shroud. A few other minor items like the head is proportionally too small (head added on in another exposure) and the back image is larger than the front by about two inches. Still a marvel, but not a religious one.