The reason is that Singapore is selfish and refuses to take the responsibilities of a developed country. There is no mechanism in UN system to force countries to be classified as developed. Of course there should be automatic mechanism to graduate countries to developed status.
Why did siddhartha gautama leave home at age 30? a. to find answers to his questions about human life b. to establish a religion that was better than hinduism c. to those who felt pain and suffering in life d. to spread his new religious ideas to other people