My theory (which is not popular with others on this Board) for this mass exodus is twofold: US porn productions have become so extreme it turns off many American-raised females who are expected to endure the rough and degrading sex; plus there appears to be a never-ending supply of eastern european and asian girls who are willing to perform in these extreme sexvids...and therefore the producers/directors will not soften the content.
The average "career" of female performers has gone from approximately six or eight years down to two years.
[Link]