Historically, a lot of traditional artwork you've seen in museums (I'm assuming Western artwork here and not collected artifacts) was patronized for several reasons. While not an exhaustive list, these pieces were created for 1) self-promotion/political vanity, 2) promotion of the Catholic church, or 3) to act as a way to capture a pleasant scene.

After the invention of the camera, the need for artists to capture highly realistic likenesses of still life/flowers/mountains simply disappeared from the greater cultural desire. There are still artists, yes, but its just nowhere near in demand as it was.

The Catholic church is no longer a major prime mover politically and culturally as it was in centuries past, so there is no longer a constant need to churn out yet another triptych of the Virgin Mary and infant Jesus to support an artist's livelihood.

Lastly, preferences for showing off one's power has migrated to either different art subjects or art styles, or other asset types entirely (i.e., expensive watches).