White people just aren't feeling her like that. Her attempts to appeal to both white and black audiences at the same time have been very weak, and everyone sees through it. Her music is also too explicit to generate the type of sells Katy Perry and Lady Gaga get from the tween demographic. So she's stuck trying to appeal to all groups and the result is a product as schizophrenic as the Roman character she portrays.
She seems to think she is entitled to the sells Gaga, Perry, and Lil Wayne generate - and throws fits when she doesn't sell well. Well she'll continue to under perform based on the standard her first album set as that fickle audience continues to move back and forth to various other pop acts. This is why I don't believe her "barbie" movement is real. They aren't the loyal army Gaga has with her "monsters" nor are they as powerful as all the tween/teen girls Katy Perry has. Also her gay fanbase seems rather low imo; gay dudes buy a lot of albums, and they aren't checking for Nicki like that.
In short I think she's done, brehs. Azelia Banks is already slowly bubbling up, she has the gay dudes on deck (pause), etc. I don't think Banks will become some huge star, but she'll probably eclipse Nicki soon enough. And if not I'm sure there's a record label out there right now crafting some light skinned black chick's image for the big leagues.