Why? The underlying business model of "being a middleman and take a 30% cut" seems pretty solid. Is it because nobody would be musicians? This almost sounds like "nobody goes there anymore, it's too crowded". If nobody wants to produce music because the ROI is too low, then some musicians will drop out, and the ROI for the remaining musicians will go up because there's less competition. The only way this will fail is if people aren't willing to pay any amount for music, but that seems unlikely.
because the streaming business model and the convenience of having any song from
any artist in the palm of your hand has canabalized any and all other possibilities for musicians that would otherwise pay better.
you have a few people/fans that insist on paying for the music in platforms like bandcamp, but thats uncommon.
from the user's point of view, it's the perfect world, where he pays a little to have access to the entire musical catalog on the planet.