Because Brave is now the default web browser packaged with ZorinOS.
Before Z17.3, Firefox was the default browser, but because of Mozilla's actions this year, (re: The Buzz around Mozilla's New Terms of Use - #59 by AZorin ) the dev's decided to move to Brave.
I personally prefer to use Firefox and LibreWolf to any Chromium based browsers.
It is a shame the devs do not offer a choice.