SARS is extinct in the wild and MERS infect no more than a few dozen people every year. Besides, you can't exactly challenge people who have been vaccinated with these viruses since the fatality rate is so high. Covid-19, on the other hand, is relatively harmless to young, fit people (meaning you can use them as test subjects), and the rate of infection in countries like Brazil should allow for very extensive phase 3 observational studies in a short time.
Like I mentioned before, if the, say, AstraZeneca vaccine can neutralize the virus before it gets to the lungs, even if you get sick, then that's mission accomplished for the time being. Billions are on the line and the putative vaccines number in the hundreds already, so something will definitely get through before long.
