COVID-19 is spread predominantly through droplets. The design of face shields typically leaves a gap between the face shield and the face. Masks that are worn closely and completely over the nose and mouth do not have such gaps. During the circuit breaker, face shields were allowed to be worn, in place of masks.
Note: With the re-opening of our economy and society, we can expect more activities and close contact amongst people, including on public transport. So masks will now be required as the default.