same as why rock and roll is not associated with rock and roll today, or country music is not associated with Country music today.
they have changed, evolved, or de-evolved, depends on how you look at it. there has been no R&B since the late 70s, it's more rap,Bass,Pop music than R&B, the name is the same, but the artist have changed there styles.
same with Rock and roll, that was dead by 1968, and todays rock has nothing at all in common with what rock and roll was about, they did try giving it new evolved labels, such as rock, and alternative rock, but it is not rock and roll, and should have it's own name now.
Country to has evolved so much, that most anything after 1985 is so far removed from " original country music" , that it's more of a child of alternative rock bands, who wear cowboy hats, which where trashed by original country artist in the early 1960s
an Jazz had done the same, but I've gone on long enough.
they should call the new form of R&B something else, just like automakers who use the same names for cars make 40 years ago, and they are nothing at all alike in shape or style.