Yeah, people listen to music more often than they go to art galleries - so what?
So music has greater mass appeal than visual art. Most popular singers and musicians didn't study music at university. It's not academized in the way that art is.
And I'd beg to differ in saying that pop music was invented in the fifties. Popular, folk music (as in music of the folk) has been around as long as art has I reckon.
Yeah of course, but I specifically meant pop music in the sense of the super-spectrum that derives largely from black American R'n'B and soul music of the 1950s and gave rise, via rock'n'roll, to basically all modern guitar music on one hand, and, via funk and disco, to hip-hop and dance music on the other (with a secondary stream running from reggae and ska through dub and dancehall to hardcore, jungle, grime and so on).
So yeah, there was popular music a hundred, two hundred, however many hundred years ago (as distinct from classical music), but is there really much continuity between what (say) working-class Londoners listened to a century ago and what they listen to now?
But even if it hasn't, again, so what?
Well my very tentative and probably wrong thesis is this: that modern art (be it conceptual or not) is the latest incarnation of a highly academic, Establishment-sponsored tradition of high (as opposed to folk) art that goes back centuries, whereas pop music (in the modern sense) derives largely from various forms that became popular with young people, first in America and later elsewhere, about sixty years ago. So modern visual art has simply had that much longer for people to have ideas.
My other main point is that self-consciously challenging music is not mainstream in the way that self-consciously challenging art is. If you can make a very broad and general equivalence between a pop song you can dance and sing along to and a painting that most people would consider 'nice' then you can make a corresponding one between very 'unmusical' music and very abstruse art. A room with a light turning on and off won the Turner Prize, but Merzbow does not win Grammy Awards.