Slacker didn't make it because Slacker is as close to a traditional radio station as you can get. Their biz-dev SVP is an old school (but not old!) college radio promo guy, who I worked with. They are old MusicMatch and Rio guys that run it.

It's not sexy, it needs some work but its what it is and doing OK.

I guess to the point, it's not a big idea. It solves the pain the author describes, but that's a pain that was previously solved, is still solved in some places, and they just kept that solution the same.

My point is that given the topic of the essay these services would seem to at least warrant a mention, if even in passing to denote their irrelevance.