[wwweston]

I think it's probably as simple as two things:

(1) Video content engages a wider audience and commands more focus than audio content

(2) Video advertisements dropped into video content are more effective than audio advertisements.

But that's a guess with no empirical base.

[myusernameisok]

Radio stations still exist and they survive primarily on audio advertisements. That's also without any of the internet fingerprinting which can be used to cater advertisements and potentially make Soundcloud's advertisements more lucrative.