Then why is it always compared favorably to X over network, which does exactly the same thing (sending graphics primitives over the network, although these days mostly having to throw over complete frame buffers because everything wants to draw itself)?

1) RDP has better compression.

2) When times changed, RDP adapted, X did not. No one uses those old graphics primitives anymore, and X's network protocol failed to keep up with the new reality of client-side rendering. So now that protocol sucks for remoting GUIs.

Yep. For many applications, RDP on a LAN is literally indistinguishable from a local session. Sadly, nothing I've tried on the Linux desktop comes close to this performance. One of the few areas where the Linux desktop is 15 years behind.

To add to this a bit, another area where the "X ecosystem" didn't keep up is with toolkits. X11 is a very asynchronous protocol, but this is difficult to work with for toolkits so they synchronize most things, making it slow over high latency links (but unnoticeable over low latency links, like UNIX domain sockets on the local host).