

One reason is, that there is no guarantee that the forks are safe to use. Any claims for “privacy-focused”, “secure”, etc should be considered marketing terms until they are somehow sufficiently proven to be true. For example, Zen Browser is one of the many Firefox forks. Its developer has made some major mistakes and otherwise behaved in questionable manner, and have basically shown to be an inexperienced developer.
https://github.com/zen-browser/desktop/issues/5947
https://github.com/zen-browser/desktop/pull/927
I just use Firefox, so I guess that one. I have meant to look into Vivaldi and Mullvad, and perhaps LibreWolf.