This may be a consequence of the properties of the creations. People who want to listen to a concerto will share the experience with many others, and typically won’t be able to buy it all just for themselves. People who want to be involved in the contemporary art world can and often do buy physical works of art for themselves, and it doesn’t matter whether or not the artist could convince a theater of people to pay to sit down together and appreciate it for 30 minutes. If art patrons want to buy physical pieces for themselves, you need a bigger pool of artists to serve them; but if you want to fill opera houses, it’s better to have a few artists that everybody has agreed to like.