why is Capetown called the Mother city?
Because it was the first European settlement in (what is now) South Africa, and hence the “mother” of all the rest.
Incidentally, it’s “Cape Town” - two words.
edit: There’s also the joke explanation that it takes nine months to get anything done in Cape Town.
Sorry to double post, but: I have recently learned that in Ancient Greek, “metropolis” literally means “mother city”. Not particularly relevant, just a related fact.