Why was South Africa the last area of the British Empire in Africa to gain its independence (in 1994)?
It is really not accurate to say that South Africa became independent in 1994. The Republic of South Africa was formed in 1961 and South Africa had been relatively autonomous ever since it became the Union of South Africa in 1910. What happened in 1994 was that apartheid ended and the country became a country in which the black majority could rule.
The reason why it took so long for black majority rule to arise was that South Africa had the largest white population of any of Britain's African colonies. These white South Africans had been ruling the country and seeing it as their native land for close to a century. They did not (for the most part) see any reason why they should give the black South Africans a part in ruling the country.
In other British colonies, there were a relatively few whites who actually lived in the colonies on a permanent basis. South Africa had a large white population and that white population had been running the country on a fairly independent basis since 1910. They did not want to give up their power or their way of life so they put off giving blacks political rights for as long as they could.