I'm going to disagree with you a bit here Justin. I think it really has been just the opposite of what you state above. "Rock and roll" in it's early days (the days of Little Richard, early Chuck Berry and early Elvis) encompassed most "popular" music. By "popular", I mean music listened to by "young people".
Beginning in the 70's, "rock and roll" began to be thought of as music that featured out-front guitars played mostly by long haired white guys exclusively. IMO, radio and the rock media (Rolling Stone especially) was to blame for much of this with the creation of FM "rock" stations that played only the aforementioned type music and identified it, and only "it", as "true" rock and roll. Now, many people particularly those born during and after the 70's think that rock and roll is this narrowly defined thing. It really wasn't that way through the 50's and 60's.
I completely agree with that, so I guess I didn't express well the point I was trying to make. Up through the '60s you had a comparatively small group of artists making "meaningful" rock and roll --as opposed to copycats and one-hit-wonders -- and it was stylistically pretty broad. It just seems that as the eligible artists get into the 1970s and '80s a more narrow view has been taken. I don't know what the formula is, but there are loads of artists who made groundbreaking, innovative, consistent albums who have been passed over year after year. Guns N' Roses, what, five albums and they get in the first year eligible, while Black Sabbath -- just an example -- was passed for 10 years? And Johnny Cash and Dusty Springfield are "in" but Merle Haggard and Dionne Warwick are "out"? I don't get it and never will, and don't give a damn for the "honor" the HoF is supposed to be.