There are some metal phones out there now. The iphone and M9 right off the top of my head.My guess would be that using a metal rear panel changes the antenna dynamics considerably and what works for plastic or leather may not work for metal. I'd love to see one as well but considering this potential problem, understand why they may choose to omit the offering.
My greatest concern with Moto is that it's best days are behind it now that Lenovo is running the show. Lenovo phones follow a distinctly different philosophy and I'm just worried that they are going to steer the Motorola 'ship' in an entirely different direction. I certainly hope I'm wrong.
It will be interesting to see what happens with Lenovo at the helm. Don't know much of Moto will be left in the next couple of transition years ahead.