Is simply because almost 38% of Singapore workforce is foreign and even work permit holders make up 27.4% of Singapore workforce, the majority of which may not even be Singaporean if PRs were instead counted as foreigners to begin with.
Imagine if a minimum wage law was passed, all these foreigners will riot if they were not also paid that amount since costs of living applies generally to all (foreigners don't get NTUC discounts nor healthcare subsidies).
To exclude foreigners from such minimum wage laws would also not sit well within diplomatic circles, as if the law here were so biased that important legislature on work benefits were skewed towards solely a certain strata.
Thus Singapore government resorts to workfare and progressive wage model to subsidise Singaporean wages albeit very grudgingly and with an overpriced and bloated bureaucracy to do so.
My suggestion is that PAP government roll back GST or remove it totally and reduce the MOM bureaucracy by half or more with flat work levies for ALL foreigners whatever their salary or job. The amount of $$$ yielded from these new levies and the reduced headcount at MOM should possibly cover some if not all of GST collected and maybe some more if that would make Singaporean workers and their foreign colleagues at work happier with a fairer and more just employment arrangement and thus make Singapore made products more innovative (better) and tourist more attracted to visit Singapore for it's reputation as a happy place rather than as a centrally managed economy with a novelty value the likes of North Korea, just to satisfy their lust for a sense of Schedenfrude, seeing people work their guts out here.
With a universal and uniform foreign worker levy, it may be estimated that the bureaucracy of the top heavy MOM workforce can probably be reduced by 80% upwards with much government budget costs savings as a result.
With a uniform foreign worker levy system, it may then be POSSIBLE to implement a minimum wage system on the basis that the both the worker and his employer are responsible to pay the levy which can be estimated to be approx $600 and adjusted as necessary in future. Thus, it might not be economical for a foreigner to accept construction work earning $1400/month if what is left after deduction for employment levy and housing costs etc does not leave him much to spend on. This uniform and universal foreign worker levy system will also significantly reduce structural unemployment in Singapore, much of which were caused by distortions created by the tiered foreign worker levy system to begin with. This existing populism driven tiered foreign worker levy only reaffirms social stereotypes and makes Singapore susceptible to international labour demand pressures since foreigners would leave Singapore for greener pastures with no nacent Singaporean workforce ability to cover for their departure, likewise the significant structural unemployment of some PMETs here when foreigners land onshore in large numbers, with the same skillsets and accepting the same PMET jobs at lower salary levels. Instead of helping Singaporeans to have good jobs, MOM has in the long run, by pandering to social stereotypes, instead caused many Singaporeans to lose their jobs and worsened the situation by endlessly enlarging it's own bureaucratic foothold and siphoned increasing levels of government budget into it's own autocratic, misled and delusional cause.