This probably doesn't sum up everyone, but I've 
seen it often enough. A person discovers 
something, say anime, enters an obsessive stage, 
can't seem to get enough, burns out and drops out 
of the scene. For others its just a change in 
career, life, or finances.

Also, quiting is hard. Most will proably relapse 
at some point and turn up back here again.

Just my thoughts..