because they have completely different water chemistry needs
sa/ca cichlids do best in 6.0-7.0 ph, while africans require a ph of 8.0-9.0.
it is not wise, nore is it responsible fish keeping..keeping sa/ca cichlids at a 9.0 ph will cause problems in the long run, same with keeping africans in a ph of 6.0.
they are from different parts of the world for a reason, if you want to keep sa/ca's and africans, buy an extra tank and use it for africans..dont mix