Politics of course.
It's not so much that it will be different than the other rifles like the Tavor or RA; just that there is a higher possibility of it being derailed than the .308. I would guess that is why they tested the waters with the .308.
If there were people within the process that wanted the .308 restricted, they will certainly be there again when it come to the .223. An election is coming as well which will make politicians jumpy about approving a cartridge better known for use in military applications than for traditional type hunting (subsistence deer hunting for example).