Anyone who has trained for awhile will see people quit training. Stick around a little longer and you will see dan grades quit too. Indeed, many, or maybe even most, quit. Many are of dubious talent, but many of high talent also quit.
I think, the #1 reason why people quit is worsening skill - I have seen this. Simply, time takes its toll on a depleted training programme and one day they wake up with perceptably less skill, perhaps combined with one or two 'events' in the dojo that make it clear their skill no longer matches time served or rank. However, a few bumble along, and even continuing up the gradding ladder - I have seen this too.
The #2 reason explains why people of talent quit but I am not sure I have the answer. Any ideas?