The reason is simply that people don't play it enough. I once tried to do draft 5 and me and my friends waited 7+ minutes, and got paired against 2 golds, 1 silver, 1 plat, and 1 diamond. We were all bronze or unranked. But matchmaking had no other choice cause there was no one else in the system. Not to mention the rampant bots (usually on beginner mode but still).
I think it would be cool if they had some LCS hot shots play dominion to bring it up in popularity. Maybe then this idea might be worth it. Otherwise, i don't see the many current fans of the LCS staying to watch the pro's play this part of the game that some of them haven't even tried.
Yes. I watch the LCS.
Yes. I enjoy dabbling on dominion and would like to try it ranked.
But unless they get more people on it, its simply not worth.