Not enough people want to pay for VDI basically, certainly compared to how much it costs to maintain it. SPICE at the time that we abandoned it "only" supported 2D and video compression, which is complicated enough, but everyone wanted 3D support which would have added another labyrinthine stack on top.

Also the code at the time was a massive security nightmare, written with no validation of fields in the network protocol. It was unmarshalling stuff off the network and sending it directly to X11 libs without even bounds checking. (I sincerely hope this has improved.)

With that said, I wish the project well. SPICE is a much better user experience than VNC, especially if you're looking for something that supports audio, video and USB passthru.