Think the main limitation is size, Dracthyr already push the limit of visual real estate. A drakonid scaled down to the size of even a Tauren would still look silly and be absent of wings which I think were important to want to include, and the drake models are just right out.
Also, do you have any idea who reluctant Blizz is to make new models and animation rigs? I’m sure they would have loved to just use any existing dragon adjacent model and call it a day. It took a lot more effort to make Dracthyr.