Having an implementation of NX from a responsive group is a step in seriously evaluating the protocol. I wouldn't be surprised if it were pulled into X (as a module) someday if people step up, push it, and shepherd the protocol and implementation. The NoMachine folks had no reason to do so; that wasn't their focus at all.
However, to many the critters have left the barn on X over the network. DRM. Fonts. There's enough pain to make people hesitate.