There are a few likely reasons as followed:
1. Silient Protest against the Gov due to recent price hike and unrest. Gov out of touch with the pple. Local losing confidence.
2. Most people are jobless , not enough $$$ to buy bread liao where got enough to buy flag
3. Some went oversea esp JB for tour during this period. Noticable at the causeway.
4. More and more Foreigners in singapore. It's not their country's birthday. Why should they hang flag anyway.
5. More and more singaporean migrated.
I don't see many Flags on HBDs.... how come?