Previously, this was arguably the best blue gem in the game for melee DPS classes. With the introduction of the Heroic gems no longer being Unique in patch 2.4, it was a little exciting to be able to slot more than one of these at a time, even if blues aren't the most desirable color for physical damage dealers. Now, the Shifting Tanzanite is still desired, but not necessarily to fill that same role or by the same people. One of the few highly sought after Heroic gems has been gutted.
Why? Nobody knows but Blizzard. One of my theories is these Heroic gems are not meant to be a "best in slot" but an alternative option, even now that you can socket multiples in your armor. It probably won't bother too many melee DPS because as good as that gem was, it was only the best blue for melee. Most melee do not stack blues at all. Tanks do stack blues, however. While this gem has become a little better for the tanks, it most likely does not outweigh Solid Stars of Elune and most certainly is not better than Solid Empyrean Sapphires for the raiders among us.