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.  
