Why are Nigeria and Botswana classified as LDCs?
I assume that you are using "LDC" to mean "less developed country." The term "LDC" is also used to mean "least developed country." The UN keeps an official list of such countries, but neither Nigeria nor Botswana is on it.
Nigeria and Botswana are seen as less developed countries because they do not have particularly high levels of income or of other measures of development such as literacy or health. For example, the CIA World Factbook shows us that Nigeria is # 182 out of 229 countries in the world for GDP per capita. Nigeria also has only 60% literacy among its adults. Botswana's average life expectancy at birth is # 193 out of 223 and Nigeria's is #221.
These statistics show that these two countries lag behind the rest of the world in important ways. This is why they are seen as "less developed."