A bit of it had to do with the SCO lawsuits, it had a freezing effect on BSD and it's adoption numbers dropped way behind Linux after that and never really recovered.
https://en.wikipedia.org/wiki/SCO%E2%80%93Linux_disputes
Earlier a USL (Unix System Lab) lawsuit went after some BSD stuff:
https://en.wikipedia.org/wiki/UNIX_System_Laboratories,_Inc.....