I agree with the guess...but also point out that the HTC and Samsung devices with themes also have Material Design.
With TouchWiz and Sense, Samsung and HTC have always diverged from standard AOSP/Nexus designs. Since Moto was bought by Google and they had time to release the Moto X, they have stuck with stock, and I hope they aren't spending a lot of time trying to implement theming in the Lollipop upgrade at this point. If they are going to go that direction, I think they'd do it with a new phone to introduce it. I hope that they don't - one of the things that made me buy Moto was their intention to stick with stock Android.
Replacement launchers like Nova Prime add plenty of opportunities to use icon theme packs. You don't need to wait to make changes like that right now.