As I have said before, I think it comes down to the custom Moto Maker options; that's the one common denominator here.
Moto X 2013: No wireless charging, several custom back plate materials (plastic, fiber, wood, leather)
Droid Ultra & Droid Maxx (released in 2013): Wireless charging, no custom back material
Moto X 2014: No wireless charging, several custom back plate materials (plastic, fiber, wood, leather)
Droid Turbo (released in 2014): Wireless charging, no custom back material
Moto X 2015: No wireless charging, several custom back plate materials (plastic, fiber, wood, leather)
Droid ??? (2015): Unknown
The common theme is the custom wood and leather back plate materials. Maybe they are not compatible with wireless charging...I don't know...I am not a phone engineer. But I would gladly take a phone I can completely customize myself on the Moto Maker website and not be like every other Moto X on the planet and live without wireless charging than have a standard color scheme phone that has built in wireless charging. That's the reason I have 3 Moto X's in my family.