They probably never developed a SARS vaccine because it didn't turn into a major worldwide pandemic. There's lots of diseases that don't have vaccines mainly because it's seen as not worth the effort or cost. SARS and MERS looked scary at first but never got this big.
As an addition here, look at Ebola. It was perfectly feasible to create a vaccine, but no one bothered as long as it was confined to Africa. As soon as white people in western nations started getting it, then suddenly there was a huge effort to create a vaccine.
