Over 1/3 of the population of Singapore (5.7 Mill.) are non-citizens (PR, on Employment Pass, Singpass holders etc)
https://www.singstat.gov.sg/find-data/search-by-theme/population/population-and-population-structure/latest-data
Another 1/3 may be under 18 Y.O., thus not eligible to vote.
There are also a small minority of citizens that is not eligible for other reasons. (Mental incapability etc.)