By all accounts the vaccines were developed in record time. Science has advanced rapidly on that front and computers weren’t around to help with the Spanish flu a hundred years ago (obviously).
There were also a number of factors in the case of Covid that’s helped:
1. Highly contagious but relatively low lethality.
2. Advances in computer tech enabling faster time to a vaccine.
3. Better medical care.
But for consideration SARS and MERS had much higher death rates:
Three coronaviruses can cause severe respiratory illness in people. One causes COVID-19, and the other two cause SARS and MERS. Learn how they differ here.
www.medicalnewstoday.com
My layman’s reading of my link indicates to me that each burned itself out quicker as it were as in the case of MERS people died at a much higher rate than Covid.