It may be a case of German government's right hand not knowing what German government's left hand is doing. An open source distributed framework can't be coerced into routing its messages through a central server for surveillance.
An open push framework only makes sense (for battery optimization at least) if it's centralized, and all applications make use of it.
Sure, from the perspective of a single device. But that doesn't mean all devices have to use the same push server.