While Nelson Mandela was in jail, what was happening in South Africa that made Mandela relevant?

Mandela spent 27 years in jail, which was a long time. I'm curious about what was happening in South Africa during that period. Why didn't another leader replace him during his imprisonment? Was Mandela still leading the movement from jail, and if so, how? How did the people of South Africa continue to trust him when he couldn't do so much from jail? How did his influence on society persist while he was out of sight for such a long time?

Please correct me where I am wrong.

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    Documenting preliminary research will improve both the probability of an answer and the quality of the answer(s). The absence of prior research, and a reason why Wikipedia doesn't suffice will probably cause close votes.
    – MCW
    Commented May 26 at 20:07
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    Have you checked his Wikipedia Page?
    – Steve Bird
    Commented May 26 at 20:08
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    Oliver Tambo was capably looking after the shop, didn't have the same charisma or any desire to take over, and Mandela had gone down like a hero. Despite the hardship of prison, he did not have to dirty himself with the war against the regime while in gaol, and had the credibility for a fresh start once he had been released.
    – Ne Mo
    Commented May 26 at 23:10
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    I don't know the situation locally, but internationally in the 1980's Bishop Desmond Tutu was a far more prominent anti-apartheid figure. The first time I ever even heard of of Mandela was Bishop Tutu arguing for his release. It probably helps to stay relevant when a recent Nobel Peace Prize winner is putting your name in the news as the leader.
    – T.E.D.
    Commented May 27 at 2:56