Why did hong kong fall so quickly?

Answer 1

The Qing army was first defeated and colonized by United Kingdom for a century due to the opium war. Japan later took control of it, and then United Kingdom was not powerful enough to stop them.

Answer 2

Hong Kong fell, first because the defenders were not expecting an attack. Second the Japanese outnumbered the defenders more than 2 to 1, and built up an overwhelming advantage in artillery firepower and air support. The Japanese simply pummeled the city, then stormed it with infantry, until it surrendered. These factors contributed to the collapse of resistance at Hong Kong after only 17 days of fighting.