It certainly could be overpowered in 5th, so I'm glad to see it gone for that reason. Also because it always kind of offended my sense of how sophisticated magic had yet become in the sixth world. Most other magic was either more ephermeral (illusions, detection), or fairly simple applications of energy (attack spells, barriers, levitation, etc). To me the shapechange and critterform spells were just so much more advanced transformation (and the shape material and a couple of other spells just a little bit behind).
I have no idea why they were left out, but fun though they could be, I'm glad they didn't make the cut.