The original SARS had a much higher mortality rate, which meant that trace-and-isolate responses actually had a chance of working. The mortality rate of Covid-19 is nowhere near that of SARS, which actually allows it to kill more people in the end because it can spread stealthily, and no lockdown or trace-and-isolate has worked. There are also more reports of animal reservoirs for Covid-19 than for SARS.