The main reason for this in my opinion is that the fear free software had of permissive licenses in the 90s by an large didn't come true. The GPL is excellent to guard against the sort of embrace, extend and close that the GPL prevented when it came to GCC, Emacs etc. back in its day, but would likely have otherwise happened. Times are different now, and while there's still some of that going on (e.g. Sony with FreeB…
This is perhaps due in part to the impact of widespread use of the GPL within the then-emerging FOSS community.