BBUS, page-6577

In both cases by the time the first human trial of a possible SARS or MERS vaccine was conducted, the epidemic was over and research into other diseases were given priority, so a vaccine solution for SARS and MERS were shelved.

I think you will find in both instances there were delays in researchers getting the genomic sequence of those Coronaviruses, to then be able to develop the antigens.

With COVID-19 the researchers reacted quite quickly and governments have thrown a lot money in the necessary research.