As people have said, the "oldies" have withstood the test of time. There's many bands from the 70's that no one has heard of, but the good ones survived.
For some context, I'm only 17, I'm a metalhead, and while I enjoy a lot of new Metal bands, I do think the older ones are better (Dream Theater, Metallica, Judas Priest, Iron Maiden, Dio, Black Sabbath, Cannibal Corpse etc.)
For me, and it seems a fair few others, my derision with mainstream music today is that it's very manufactured and simple. I have no problem with people who listen to mainstream music, but I like my music to have complexity, without autotuning the vocals, with technical, amazing guitar/bass/drums/keyboard. I'll start criticising if someone starts up with the "Metal is just noise" crap, but otherwise I don't really care, to each their own.
So really, many people (me included) don't like how "fake" mainstream music is becoming, and it makes sense that a lot of people would like old bands since they have fans accumulated over decades, as well as obviously being pretty good/liked since they're still known today.