Not only did the Steam Machine suffer from a LONG period between announcement and release (2+ years I believe, even longer if you count the leaks of it, which would probably be three and a half years), but it lacked exposure. The most likely people to buy a Steam Machine are people who already own a gaming PC. Not only because those are the people who know about it, but those are the people familiar enough with the platform to circumvent its many issues. About the only advertisement I heard of were small tables set in the corner of select Gamestops. That's definitely not bad advertisement, but it's not enough, imo. It'll sell some units, but not as much as actual marketing would.
And then there was the issue of the things being too damn expensive, the reportedly bad performance of SteamOS, the lukewarm response to the controller, I can't see it not failing. But, if during that time, SEGA hops on and releases their own Machine at a reasonable cost, I'll definitely be interested.