Severe Acute Respiratory Syndrome (SARS) is a disease caused by SARS-CoV—the "SARS CoronaVirus".
We now have a second disease caused by a second coronavirus, which we call SARS-CoV-2.
So why is the disease named COVID-19 now, instead of SARS-2 (or SARS-19 if you wanted the year)?
Disease are officially named by the WHO, while viruses are by the International Committee on Taxonomy of Viruses (ICTV).
The WHO has stated
From a risk communications perspective, using the name SARS can have unintended consequences in terms of creating unnecessary fear for some populations, especially in Asia which was worst affected by the SARS outbreak in 2003.
For that reason and others, WHO has begun referring to the virus as “the virus responsible for COVID-19” or “the COVID-19 virus” when communicating with the public. Neither of these designations are intended as replacements for the official name of the virus as agreed by the ICTV.
Both the disease and the virus have been officially named February 11th 2020, while it has been declared a pandemic by the WHO only March 11th 2020. In hindsight, it is debatable whether „using the name SARS[-2] can have unintended consequences in terms of creating unnecessary fear for some populations“ is true and whether this fear would have been „unnecessary“. But this is the reason we ended up with a different nomenclature of the disease and the virus.
It might also be worth pointing out that many viruses are not named similar to their respective diseases: HIV <-> AIDS; HPV <-> Cervical Cancer / Genital warts
