Damn, it seems my earlier guess was right... I found this on a thread on another forum about Laura from nubiles/LauraLovesKatrina, from July this year:
At first I thought the intention was that they didn't want to compete with their own new site (LLK). But Katrina is still on Nubiles.
Guess it is some kind of license issue. Apart from Laura also Emily, Briana, Selena (Sema), Samara, Sonya, Milena and perhaps some others disappeared from the Nubiles site as well.
Guess getting hold of those full sets will be pretty difficult now