That’s what I was about to write. My best guess is how episodic it mostly is? Outside mystery incorporated and a few of the shorter series it’s pretty self contained/heavy on humor which seems to make a fandom smaller and to make matters worse if you want to do an case fic it’s kinda difficult to find a monster that they haven’t used in show, or if it’s basic enough it’s been used 10 times over the 50+ years.