I thought that no vaccine was developed for SARS and MERS because the diseases disappeared, so : not enough volunteers to test the vaccines, and also companies lost interest.

from an npr interview[1] :

BRIGHT: Unfortunately, when the virus disappeared, the funding tended to disappear with it. And the companies that were making a SARS virus vaccine lost interest and shipped it back to their more profitable vaccines.

[1] : https://www.npr.org/2020/03/13/815307821/planet-money-why-th...