I think it’s already mention but yes, it’s due to the possibility of false negatives from the test.
Mask are already mandated in every public space in Singapore even on land, and the population has basically adjusted to that fact already, so it isn’t that big a deal to most singaporeans anymore.
Also, take into consideration that Singapore as a country is similar/smaller in size than an average City. With no real options for travel right now besides cruising, I think most people wouldn’t mind wearing a mask as long as they can leave Singapore for a few days. Can’t exactly take a weekend drive to the countryside ?