Some don't want to film more degrading scene or increase the quantity it of titles to make up the difference. Some even felt like they made enough and decide to settle down and pursue other things in show biz or their private life since they're e older and always getting typecast. More and more new girls join every month and not all veteran Diva gets perk. Those that comeback usually are desperate in need for some money after all they can't get hire for decent jobs without a college degree. Maria and sora got lucky retaining their fame and found their success and other halves.1. less income -> less money to make vids -> much more selective to make retirement vid
2. more and more of them come and go and comeback, so retirement / final / farewell vids its not necessary anymore