It seems like many of the hit rock and roll songs around the 50s and early 60s were originally written and performed by one artist and then popularized by another.
Nowadays it seems like re-using other groups songs and "old standards" are more rare in mainstream pop music. When did the shift occur to each artist being expected to perform mostly original material?
I addressed a similar question a few months ago here. Other repliers to that question also filled in other reasons for the change quite well.