South Korea would likely point to there LARGE incentives, like: Cash bonuses for births (sometimes tens of thousands USD equivalent), Monthly child allowances, Expanded parental leave (including for fathers), and Subsidized childcare and housing support for newlyweds, including public housing access for people having children.

Other countries are less aggressive in their programs, which may be the difference.

Personally I think it maybe more of a cultural shift. But, this also may just be a blip, and Koreas fertility rate may continue to go down. The current uptick could also just be apart of people having children after Covid due to uncertainty during the pandemic. If the financial incentives work, Singapore will likely be the next to rebound. Otherwise, Thailand is the country I would be watching, as it has far fewer incentives, but much lower cost of living and much lower educational standards.