My theory is Wayland happened. As SPICE doesn't work that well through it. I would assume it's another case of a "niche" the Wayland protocol didn't account for, however.
Interesting theory. Any idea on why SPICE wouldn't work well through it? I don't have recall running into any issues with it.
A more Wayland-oriented remote desktop protocol would probably make for an even better VNC alternative, but I don't really know why SPICE never got the uptake it deserved.