Even the countries that have attained the prime position as compared to other nation still they need to constantly qualify as a developing country, becuase without constant improvement they might fall back.
Question comes how does one define a developing nation, is it economic growth? OR per capita income? OR happiness index of citizens? OR increase in education/special skills? or something else?
All the point which i have highlighted, are the cause and consequence of economic growth of a country. if the per capita income of a person is high, it leads to the expansion of his needs, which are fulfilled by resources, now theses resources cannot be used raw, they are processed, and to process them industries and factories are opened up, and this is how the process leads to economic growth. Increase in education or investment in education leads to more number of children going to school, which returns higher yields in the future(i.e they get better jobs, and therefore better income) and thus leads to economic growth.
happiness index is the consequence of economic growth because more the economic growth, more job vacancies will be there, be it the primary sector or the tertiary sector or casual unskilled labour etc.
India is defined as a developing country because we have many natural resources like bauxite, iron ore, copper ore etc. but we have not developed the technology to process these resources and neither does the government have the money which can be invested in such projects.
but since our GDP(gross domestic product) has been rising and more job oppurtunities have opened up, we consider india as a developing nation