I think this is an economic barrier more than a scientific one (not that the science is trivial). MERS is too sporadic. SARS burnt out to quickly to substantially test anything clinically, and if you look at the scientifc literature, interest in SARS dropped pretty quickly. I’m general neither industry or investors or government funding has that much interest in Infectious disease (vs cancer for instance). Pandemic preparedness was a horrible business plan.

AFAIK there has never been much of an effort, because it is not a high priority vaccine. Based that all vaccinations can have a side effect, why would you develop a vaccine for it. And for the other SARS I suspect they were to quickly contained by other means to warrant development of a vaccine.

Vaccinating away a single cause of common cold out of many causes would seem almost pointless. I'd take it as a given that lack of seriously trying has been playing a major role in that lack of success.

Also we have a coronavirus vaccine for pets and livestock that you can buy today.