My take on why girls ultimately switch to contract:

A lot of them start out with gonzo porn to get instant fame.To get that fame, they have to do some things whch some may find uncomfortable - DP's, A2M, Creampies, black guys(I betcha quite a lot of porno stars are racists), swallowing etc. etc.

They don't like it but do it anyway because it keeps paying the bills and makes them famous. Then when a contract offer comes in, they fall over themselves to sign up. Why? Because they get a lot more money, they don't have to live from paycheck to paycheck, they don't have to do the gonzo porn things they used to do, they don't have to do inter-racial and they don't have to do the rough stuff
after a while, once they think they are well off, they quietly ease out of the industry to get married and settle down and leave their porn past behind