It could be survivorship bias. All the fake shrouds with botched images were recognised as fakes and thrown out long ago. Only this one which happens to look vaguely realistic has been kept.
Plus what must have been lost in 500 years, all photonegative pictures, fragments, descriptions, recipes, any references for such imagery etc. It is very strange that this is the only image that remained. And an image of a nude Jesus from the back. For which no other instance is known anywhere.