Well, it might be because we know why and how a rainbow is made... and we CAN make a rainbow. So far, including this attempt, we have been unable to make the Shroud.
Well, I would like to see someone attempt to make a rainbow that often stretches for miles across the skies. The purpose of the rainbow isn't for adoration. The purpose of the rainbow is to remind us of a promise of God.
And if your criteria for a relic is to imitate God's miracles, I would remind everyone that the Eygptian magicians were able to duplicate some of God's miracles with their arts. That didn't make God's miracles any less a miracle, nor did it make the magicians' art a miracle.
So far, including this attempt, we have been unable to make the Shroud.
According to this article, they have.