I consider Janet's sales to be a book. She probably would have done twice as much with heavy promo but I think she played it safe. A huge media campaign and disappointing sales would have been hard to live down.
Ciara and Tamar are proof that they do too damn much and don't focus on releasing quality music. People view them as a personality/entertainer not a singer and they don't want music from them. You can get plenty of Tamar for free on tv so why spend coins on her music?
People like Jill and D'Angelo book because people still respect them as artists.