Steven Moffat says that great villains lose their impact if they show up again and again. He's speaking specifically about Moriarty and Sherlock Holmes — and references Batman and the Joker — but the broader point stands.
Moriarty (Andrew Scott) in a still from Sherlock. Image: BBC/Hartwood Films (via Baker Street Wiki)
Speaking to Entertainment Weekly about the new season of Sherlock, and about introducing Toby Jones as a new villain, Moffat said:
You don't really need a master villain. Sherlock doesn't need an arch-enemy, even though he has the most famous arch-enemy in fiction. And we already know Moriarty. When you introduce a new villain there's more mystery about them — what's their point going to be? What are they up to? How is Sherlock is going to defeat them? You can't have the Batman vs. The Joker every week, you get depressed when the arch-enemy never seems to notice that they always lose. It just becomes predictable. There's a whole host of less-famous ones in the stories that have never been touched.
There's another thing that Moffat's leaving out: The more often we see a villain interpreted, the more we're going to end up comparing them. Moffat's other show, Doctor Who, has that in all the versions of the Master and every edition of the Daleks that keep popping up.
Plus, it's always more fun to populate a universe with many different and interesting bad guys. Batman, as Moffat referenced, has a rogues gallery that is deeper and more famous than most other heroes have. That's been a boon every time Batman has been adapted to another medium.
More villains, and more better villains — it's a lesson everyone could stand to learn.