Late stage vaccine trials are conducted by vaccinating a bunch of people and then seeing how many get infected versus a control group. So it's not just a matter of "not needed"; the trials simply couldn't be performed without an active outbreak of the disease. There was a lot of promising early-stage development.

SARS had several canditate vaccines in human trials. These were all shelved once it was erradicated.

MERS vaccine development is ongoing, with promising canditates in human trials.

Wikipedia has a brief summary of prior work on vaccines against other coronaviruses, including SARS and MERS.

https://en.wikipedia.org/wiki/COVID-19_vaccine#History

That's how these got developed so fast, take a MERS vaccine and update it.

Click the wikipedia article, and click to the History section.

From my understanding, they were started, but then both SARS and MERS burned out so fast that they couldn’t find enough candidates to test the vaccines by the time they got to trials.