Probably because the infrastructure had been left completely unmaintained and run into the ground by the ANC. By that time that the water tankers were the only option.
You can't provide water to the people in pipes when the ANC destroyed the pipes during their time in office.
By the time the DA got into office the damage was done.
You don't see that problem in Cape Town because: maintenance by the DA.
The DA just spent millions upgrading a poorer areas sewerage infrastructure so that these kinds of things don't happen. Cyril was right, the ANC can learn from the DA.