[bcgraham]

It’s driven by the nature of jokes in a long-running medium. Jokes both require and create context; this joke’s punchline is that joke’s setup. If the setup isn’t rooted in the current context, it’s nonsensical. If the punchline is in current context, it’s boring.

It’s easy for this process to feed back, especially with large casts of characters. I heard a writer once talk about “protecting” characters from some jokes, I believe to prevent a process like this.