Maybe she needs to eat more. [joke] (actually I do wonder whether she maybe lost a bit too much weight for SI2007).
Perhaps she isn't as famous because she doesn't go to lots of events like a lot of the other models. Perhaps also because she isn't a catwalk model. Perhaps also because she doesn't do more shoots for people like Maxim etc. or more TV shows and adverts (which is a shame). I think she prefers a more 'normal' life rather than lots of promotional/publicity events and that is why she isn't as famous.
The film she is in isn't easily available in the USA or UK so that means only Italian people will usually be able to watch it. Even some of the Sports Illustrated DVDs she's in aren't available or re-released in places like Amazon.
So she doesn't really do enough new things (magazine shoots, TV shows, films, etc.) and a lot of the things she has done aren't easily available to buy.
EDIT: I think she should have an official website like some of the other models (like Marisa/Petra).