I think its mostly "political", see this
for the full thread. There is now three backends, mine, KhronosGroup's and Codeplay's and I'm trying to convince everyone to collaborate. If you're interested, the TableGen backend
is the next thing that needs doing, but I'm going to wait and see if I can convince someone else to do it for me ;) (either you or authors of the other SPIR-V backends). It it alas rather boring work, although thrilling in comparison to writing the tablegen desctriptions.