We could start by cataloging the reasons people don't stick around. A few things that come to mind:
* Some people take up dancing primarily to try to meet someone. Eventually, either they do meet someone, or they give up trying.
* Some dancers eventually become no longer able to dance due to health problems. I've known a few that will still stick around, but in most cases, it seems that dancers who are unable to dance can't bear to be around dancing; it's too painful to watch others and not be able to do it. Or, maybe they can still dance somewhat, but not up to the standards that they hold themselves to, so they don't do it.
* I can think of two couples who no longer dance because circumstances of their lives have changed and they no longer have the money or time.
* Some dancers leave because the social aspect of the dance scene where they are is not to their liking. Usually, this is an age group thing.
* Sometimes people have to relocate, for job or family reasons, to an area where there is no dancing.