The decline of Buddhism has been attributed to various factors, especially the regionalisation of India after the end of the Gupta Empire (320–650 CE), which led to the loss of patronage and donations as Indian dynasties turned to the services of Hindu Brahmins.
Nigeria had much division between tribal/ethnic groups vying with one another for power, and thus were not working together for a united purpose. Nigeria's wealth of natural resources also gave them much to be fighting over as the groups each sought advantage.
Botswana, on the other hand, was one of the poorest countries in the world when it gained its independence from Britain in 1966, and it avoided infighting and political corruption like that seen in Nigeria. They have been a multi-party democracy since that time -- the longest-running multi-party democracy on the African continent. Botswana enjoyed significant economic growth also as it worked together politically.
