Here’s a much deeper answer.
Race.
Both genres have their origins in the blues. But in segregated America in the 1950s and 1960s, there were radio stations that played music by white musicians for a white audience, and by black musicians for a black audience.
Rock'n'roll is just the blues, speeded up. And while it was pioneered by black musicians, it was rebranded for a white audience by getting white musicians like Elvis Presley to play it. British musicians like Mick Fleetwood, Keith Richards and Robert Plant started off as huge fans of the blues. As one bluesman said of these young Brits "They want to play the blues so badly, and they do..." Actually, of course, they are very fine musicians. But when British rock'n'roll came back across the Atlantic, it was white musicians so got played on white stations. And we thought that the Stones and Led Zeppelin were making a new original sound, whereas in fact they were massively derivative of Chicago blues.
So rock'n'roll became the music for a white audience, and rhythm'n'blues was the same music played by black musicians for a black audience. Over time the styles diverged, but if you listen to 60s or 70s r'n'b it's much more raw and, basically, rockier, than today's cheesy bump'n'grind stuff. And a lot of today's rock, whether it's metal or indie, has lost the roll, the syncopated beat that gave the blues the groove. Personally I think both genres need to get back in touch with the blues.
But even today, the racial make up of perfomers and audience for rock and r'n'b still reflects that original segregation made by the music industry in the 1950s and 1960s.
