Ironically, BSD was already available when GNU was created. Since that is what macOS uses, I guess the question could be “why didn’t everybody just use BSD”?
I suspect two of the many reasons for GNU usage were
(1) crazy command options (it only takes one or two to ease your workflow over the what the system distro supplied, plus the same behavior for the same flags across platforms)
(2) the lawsuit (probably an understatement - it was Serious back then)
Edit: added "usage" for clarity, s/statment/statement
