In fact there are many RNA viruses and there is no reason they can't have a vaccine. In fact the flu we get every year is a good example. Influenzas are RNA viruses with a new vaccine coming out every year.
Vaccines sometimes don't even have the RNA/DNA of the organism you are trying to immunize against. (I'm just going to talk about viral vaccines - there are many other related topics). Immune response is typically targeted to proteins on the external surface of the virus. Antibodies to these proteins are not only effective in creating an immune reaction, but also are ideally protective; some antibodies will block the virus from infecting and help actively fighting the disease. That's a double win and a big reason why previously infected individuals might become immune to a disease: the antibodies in the blood are stopping the virus from acting on contact. There are no technical barriers to a vaccine.
So why don't we have a vaccine? There are literally millions of families of viruses and its not cheap to develop a vaccine and that money means nothing if the public don't accept vaccination. While SARS and MERS were a strong indicator that Coronaviruses would be a threat, in themselves they turned out to be easy enough to stop without bringing the global healthcare apparatus to bear.
Even if they did develop one, its not clear that a SARS or MERS vaccine would be effective against other cornona viruses. Even though the flu is from the same species of virus, we need a new one every year to react to how quickly it changes.
If there had been a corona vaccine available... lmk ask you a question... The MERS outbreak was 2012. The public, having lost awareness of the benefits of vaccines, are not big on seeing the value of paying even $10 for a shot and then not getting sick. You never knew it did you any good. Would you have taken one last fall? Every year for the past 5 years?