The Asian water buffalo, Bubalus bubalis, was domesticated several thousand years ago and now numbers in the hundreds of millions as a draught, dairy and meat animal. The African buffalo belongs to a different genus, Syncerus, and has never been brought into any comparable relationship with people.
Temperament is the usual explanation, and it is partly right: African buffalo are unpredictable, form strong herd bonds that do not transfer to a handler, and adults will turn on a pursuer. But the harder constraint is disease. African buffalo are a natural reservoir for foot-and-mouth virus, bovine tuberculosis and corridor disease, and contact with cattle transmits them, which is why fencing to keep buffalo and livestock apart is a standing feature of southern African veterinary policy.
One detail is often overstated. The reputation of buffalo as Africa's most dangerous large mammal comes from hunting literature, where a wounded animal is genuinely formidable, and it is not a reliable description of the animal's behaviour toward people who are not hunting it.