Erm, it's pretty simple.
In the late 90's/early 2000's, the 'in' thing was R&B music. Then it became about pure pop and dance music. We are slowly moving back into R&B music being the popular thing.
Beyonce managed to succeed throughout the pop-period because of B'Day and I Am, 2 generally very pop albums with little influences of R&B.
A lot of black females don't do pop or dance music, it's that simple. You had many black females singing R&B claiming that pop radio wouldn't play them but they'd play Adele. Erm, well pop radio is not playing R&B music right now, what do you expect? RITD, SLY, and SFTTR all had very pop tendencies that were very influenced by soul.
Erm, I hope I explained that well enough
