I think the COVID-19 virus, SARS-CoV-2, is much more infectious rather than virulent. SARS-CoV-2 binds to the ACE2 receptor on human cells with 10-20x affinity than SARS-1. This means that SARS-CoV-2 can easily infect a person, compared to other known viruses.

Coupled with the increasing evidence that SARS-CoV-2 can travel by aerosols and the indoor crowding with poor ventilation (enabling aerosol spread), it's not totally unexpected that SARS-CoV-2 has spread so widely.