https://en.wikipedia.org/wiki/Presidential_Elections_Committee

They have a committee that decides if a person is suitable enough to run for elections, based on very non-transparent rules. The current President was deemed the only one "fit enough" to run for Presidency and so she got elected without any opposition. Her selection also came with some race related controversy, because SG tries to do race balancing and so this cycle was unofficially reserved for a Malay. Halimah is only half Malay it seems (Indian father, Malay mother, officially registered as Indian) but identifies more with her Malay heritage than her Indian heritage.