their are many reasons that chicks don't stay in the porn industry.

1- they decide they like the idea of pursuing a drug habit instead

2- their boyfriend (who has sat on set shoot after shoot) decides it best for their relationship if she stops doing porn.

3-Their college careers I've heard many a girl claim that she was only doing porn to get $$$ for college. Out of these girls I really don't know how many go to school or even graduate from college

4- the reality factor, they will do a bunch of movies then when they come out someone in their lives recognize them in one and they will be so humiliated they never do porn again.

I'm sure there are other reasons as well feel free to add on to my list.