Compare the SARS-CoV-1 outbreak in 2003 with the SARS-CoV-2 outbreak now.
“The NIH scientists, from the National Institute of Allergy and Infectious Diseases’ Montana facility at Rocky Mountain Laboratories, compared how the environment affects SARS-CoV-2 and SARS-CoV-1, which causes SARS. SARS-CoV-1, like its successor now circulating across the globe, emerged from China and infected more than 8,000 people in 2002 and 2003. SARS-CoV-1 was eradicated by intensive contact tracing and case isolation measures and no cases have been detected since 2004. SARS-CoV-1 is the human coronavirus most closely related to SARS-CoV-2. In the stability study the two viruses behaved similarly, which unfortunately fails to explain why COVID-19 has become a much larger outbreak.”
https://www.sciencedaily.com/releases/2020/03/200317150116.htm
Larger?
Because China hid it. That allowed the seeding of many places due to the 14 day incubation period.
That that and if is true that there is 50% of the infections that are without symptoms, it is hard to control by normal means.