The most significant reason majority rule was delayed and not initiated sooner than its beginning in 1991-1994 was that post-colonialist English and Dutch Afrikaner citizens did not recognize black Africans as equals and citizens. In other words, the English and Boers did not want majority rule; they did not even want recognition of black citizenship.
While South Africa was always a country of minority rule by the English and Dutch Afrikaners, the 1950s brought increasingly oppositional restriction. Blacks were restricted to working and living and owning land in certain designated townships that were most often at significant distances from white towns and cities. Masses of black laborers walked or took trains before dawn into towns and cities from their outlying townships every morning, spilling over from the sidewalks and filling the streets with an all but solid wall of workers who each had to an identity Pass Book or "Dompas," also called the "Book of Life," permitting day-time travel to a whites-only town or city.
Thus one delay was the totality of disempowerment that the government practiced against blacks and other "non-whites," non-Europeans. In other words, with barriers such as were in place interfering with meeting daily needs of food and shelter through productive work, opposition to oppression was not easily fomented (i.e., stirred up).
When education, which was limited in availability and quality for non-whites in any case, was further restricted and made even more unattainable, students finally found the fuse to ignite the thus far unfomented anger of protest. After the fateful 1976 student uprising in the Soweto ghetto of Johannesburg protesting the new Afrikaner government law requiring school courses be taught in Afrikaans instead of in the traditional English, the government tried to appease this broiling anger by creating "semi-independent" homelands where blacks would be housed and of which they would be "citizens." While the government gave more liberal recognition to some "coloured" groups, like Indians, the "liberal" gestures of "citizenship" toward blacks isolated and restricted them even more.
Thus another delay was the continued ideological conceptualization of the insignificance and inferiority and non-humanity of non-whites. This ideology was apparent in the new schooling ruling that was designed to rob them further of education without the overt appearance of absolute oppression before the critical and watching world. This ideology aimed to overpower non-whites, especially blacks, and deprive them of all recognition of humanity.
Finally, in 1991, following President de Klerk's realizations of necessity and the end of a domineering reign of suppression by the post-colonial powers, apartheid was withdrawn and a new constitution drawn up, which led the way for Mandela's election in free majority rule voting.