I would guess it's a combination of many things, some that I listed below.
- Mating has historically been arranged until the last 50 or so years.
- Culture is more traditional where there's less women in the work force. Often times, partners meet each other at work.
- Standard of living is very high, market is very competitive due to focus on education, and less suitable partners that can sustain a family, thus decreasing the attractive of supply side on the male partner side for women.
- Small countries cramming people into a small land mass, making it difficult for the country to expand outward, thus must go upward. Limiting economic growth activities and income, thus requiring for highly skilled labor, increasing standard of living.
- Passiveness of the region culturally (less prone to war, maybe outside of Japan), men and women are more passive, crime rate is extremely low, and women being approached by men are also low. Hence when non locals, like americans/europeans go over and talk to a girl they find attractive, that is likely the first time in that woman's life that has happened (depending on the attractiveness).
- due to lacking amount of land to expand, real estate is ridiculously expensive
Basically, they are overworked, have a higher standard of living than 3rd world but are still poor, can't afford homes or to start a family, making many men depressed and not suitable mates, and making it difficult for women to find partners as well.
