The main audience these popstars cater for is vast and diverse, yes, but arguably also the most challenging of them all - there's usually no "extra" money sitting around and it's common knowledge how to acquire music cost-free, and so save the coins for something else. Not to mention the competition is stiff.
Also, Rih's performance skills/talent are always called into question. If she was consistent and always slaying in that regard, her album sales in the US would be a lot better. Anyways, a lot of what Scherzy said is true. There are a multitude of reasons Rih doesn't sell as well as her peers and most likely never will.
Another reason is Chris Brown the incident itself lost her a lot of fans and so did the reconciliation it's like she lost urban in 2009, lost everyone with the darkness of Rated R, gained a tiny bit of White America with LOUD, lost them with her "bad gal" image, and gained urban back when she got together with Chris