Promotion.
Artists can self-promote all they want, but they're up against thousands of other artists that are also self-promoting. Labels pay people to listen to artists, work out which ones are actually talented (or, in reality, which ones will make the most money) and focus their promotion on those artists only. The signal to noise ratio is totally different, and they can afford to put far more resources behind this smaller pool of artists. They also have a lot of experience in promotion, which up and coming bands simply don't have.
There isn't a tech startup solution to everything. Maybe you could set up a site that crowdsources this artist selection process but I wouldn't be optimistic- there is no personal reward in listening to hours upon hours of awful music. Maybe you could give those initial listeners a stake in the bands they vote for? That could be an interesting idea.
I've long thought that MySpace might yet have a life as some sort of band-centric site. Since it died a horrible death nothing has really come up to replace it.