It isn't specific to Singapore, but the entire region. In the early 19th century there was huge demand for Asian goods in the west, which led to a huge trade disparity because China had little interest in western goods. The British East India Company was able to negate that through smuggling opium to such a degree that it was emptying the Chinese coffers and caused a drug abuse epidemic, which then forced China into conflict with Britain. This led to the Treaty of Nanking starting a trend of colonial subjugation and weakening of the Qing Empire and is considered the beginning of Century of Humiliation in China.

Then you have to take the additional context of how power coalesced in many of these Asian tiger economies through strong authoritarian governments during their modernization and independence. I do not think it is a stretch to think that other nations in the region, especially those with close proximity to China, or have large Chinese populations (like Singapore), saw how unbelievably poorly this went for China, which probably contributed to the extreme measures used to prevent recreational drug contraband from being used in their societies.

Most likely hardlines in power. Much like early South Korea and Taiwan, who's people had fortunately broken out of this trap.

https://en.m.wikipedia.org/wiki/People%27s_Action_Party

Opium is important in the history of Singapore. The city was mainly developed for and by the opium trade, but also faced issues due to legal local consumption up to the 1940s. It's not what directly brought the draconian legislation in the 1970s, but it certainly contributed.

Some history on opium and Singapore:

https://biblioasia.nlb.gov.sg/vol-11/issue-3/oct-dec-2015/dr...