Consumers buy tickets to a DJ's show, DJ pays indie label, label pays artist. So three things should be clear: first, DJs aren't on Soundcloud to get signed to a major label, they are trying to drive ticket sales; second, unknown DJs don't use Soundcloud to promote themselves because ticket buyers must be local.
Finally, amateur artists don't care about developing a following like YouTube stars because their music is sold to DJs, not to listeners. Part of the appeal of a DJ is that you hear great tracks at their events that you never heard before, so they aren't going to want to buy viral Soundcloud hits that the audience has already heard.
In short, the mainstream music economy is driven by MP3 sales/streams directly to listeners. The underground music economy is mostly about event ticket sales, and Soundcloud is valuable to artists if it can drive ticket sales. That's why its current monetization strategy is mostly about telling artists which cities their fans are in. Frankly, if I were Soundcloud, my next step would be to build a ticketing platform.