Why was the country of South Africa so divided when Nelson Mandela became president in 1994?

Apartheid.

Explanation:

After WW2 the Afrikaans speaking white community, descended from Dutch settlers, gained political power and introduced apartheid. This was institutionalized segregation based supposedly on the concept of separate but equal development.

In reality it was institutionalizing and attempting to legitimate the exploitation of the black community at the hands of the minority white regime. It also discriminated against the Asian community and those of mixed race, "coloureds".

Any form of protest was ruthlessly suppressed, e.g. Sharpeville in 1960. So it was inevitable that organized opposition would become more violent. The international community did little or nothing except pay lip service as South Africa was one of the most lucrative countries for foreign investment, specifically because of apartheid.

Mandela himself along with other leading opponents of apartheid were jailed for life in 1964.