No show in TV history has a bench as deep as The Simpsons’. Over 30 years of broadcasting has given them a whole town’s worth of weird and wonderful characters to play with, and they’ve been eager to add real depth to most of the roster. Springfield residents may debut as stereotypes or quick gags, but in due course they're often given real depth.
Sometimes, though, they bring out a one and done character. They show up, they do their bit, and they leave town, never to be seen again. This may be due to a guest voice who they can’t reasonably bring back, or simply come down to the limitations of the characters - not every creation has to have a fleshed out backstory, a family history and so forth - there just aren’t enough hours in the day.
The brevity of a character’s life span does not necessarily have any bearing on the quality, though. Some of The Simpsons’ most indelible creations stuck around for no longer than the one episode - their runs were as brief as they come, but the jokes and impressions left are as memorable as any the show has had.