I have seen a lot of people leave, and the main 
reason (other than money) is probably frustration-
 most people do not have the patience to spend 
years building a nice collection.  I think a lot 
of people get into the hobby thinking that it 
will be easy to get a few great cels of their 
favorite characters (I thought it would be easy- 
HA!). After a little while they find out that 
most good cels are snapped up pretty quickly, and 
it can take months to years to find even one 
really good one, and then, sometimes, you can't 
afford it when it comes along.  
Other time, people leave because the hobby simply 
doesn't hold their interest.  I can't really say 
why I still collect, except that it's still a 
thrill to find a nice cel (especially when it's a 
good deal)and I still like flipping through my 
cel book, and looking at all the lovely 
pictures.