Why do schools insist in the language of instruction being English when majority of the children do not speak English?

I have a partner who has taught at several schools and a thing that they have picked up is that many of the children she teaches in school are Zulu HL speakers (We are in JHB).

I feel that the system is failing these children since they are not able to communicate as clearly as they could have if they spoke their home language.

I was in a position where English became my first language and I struggled when it came to expressing myself in Afrikaans papers.

It breaks my heart knowing that there will be children whose self-esteem, worth and accomplishments will be limited to the grasp of a language they do not often use.

Is this for the best or are we failing these children?