I believe it was declared a PHEIC - Public Health Emergency of International Concern - and this should be the signal that governments need to take measures for containment to prevent it from spreading.

I believe the pandemic label is when containment has failed and governments need to go into mitigation mode. Seeing as it had been relatively contained in China, South Korea, Singapore, Hong Kong, Taiwan, etc., it seemed reasonable that taking measures would be sufficient to prevent further escalation.