We’ve been talking about this one quite a lot. Having a significantly out-of-band item existing on live is always a dilemma. We either nerf it and deflate the players who had the excitement of getting it (or even worse, actively invested a lot of time and effort into getting it), or leave it in place potentially distorting future behavior. And the longer we wait, the worse it gets. So deciding usually relies on the judgment of whether the situation will taper off if we leave it alone (due to higher-level items eventually becoming available), or will get worse somehow.
In the case of Drape of Shame, the base Mythic version would potentially have been acceptable to leave alone once it was live. But the imminent arrival of Mythic Keystone Karazhan was a clear way that this particular problem would rise to a new level if we left it alone. So it was better to make a change now, even at the risk of disappointing some players who have it (they have still gotten to enjoy it for some time throughout the current tier), to avoid re-creating the same problem anew in the next tier.