Because different subgenres emerged, and "rock & roll" needed clarification. The new subgenres already had adjectives. Those same adjectives apply, to-day, and they will never become original-flavor rock & roll.

Old enough to remember when the first 'classic rock' stations started replacing 'oldies' stations. In those days, the 'classic' stations would play new music from old bands, and even brand new stuff from new bands. These days, a "1980's" station would never play a record from 1979 or 1990.