I would tend to agree with evenflow's anaylsis. Many students see black belt as an ending rather than a beginning - following the emotional high and subsequent let-down after testing for black belt, many students lose their enthusiam - especially in systems in which the time for testing increases steadily between black belt ranks.
Some students continue to attend class through momentum - by black belt, they have been coming to class for at least several years, and attending class has become a habit. Some of these students will regain their enthusiasm as time goes on, but many will drop out - after II Dan, if not sooner.
Some students move on to other styles - they remain active martial artists, but want to explore other styles. These students will often garner belts in multiple systems, and continue for long periods of time.
The students who continue because they love the martial art in which they participate for the art itself, not the rank they have (or might) attain - these are the students who show up to every possible class, who help because they want to and not because of any requirements imposed from above about involvement, who continue whether they expect to ever test again or not (due to injury, age, disability, physical debility,
etc. - any factors that prevent them from attaining further rank) - and they are, in general (at least in my opinion), the students who become the best instructors.