Interesting question..
I think it is perhaps because some bands have a certain timeless, universal appeal to them.. A universality that somehow cuts through generations..
Groups such as U2 or The Police are further examples. It's interesting, I was playing
Joshua Tree around my pre-teen son, and he asked me who they were. I explained, and his response was, "Wow, they have a really great sound, I can't believe they are so old!"
It is a little like literature, or any art form, I suspect. Some singers, artists and writers have an ability to touch people no matter their age or cultural/historical period. One could just as easily ask, "Why do we still read Shakespeare and Hemingway while others writers have come and gone, almost unheard of?"