I really don't care all that much. How many of the millions of people on Steam do anything more than boot it up or have it idle in the background? How many only installed it because it's integrated into a game they purchased a hard copy of? How many of those users use the forums to do anything more than complain when something doesn't work? How many use the community features at all? How many care enough to set up their freakin' profile?
To complain that only 15K users out of millions check out Greenlight on a regular basis (even assuming this is correct), seems like it's a logically invalid inference. There's no guarantee that you'll get X amount of users or X percentage of users. If only 15K visit, then only 15K of all of the people that use Steam for any number of other reason care enough to also spend time browsing through the mess of unproven and unknown indie titles.
So most people don't care, big whoop. Steam is a game distribution service first, DRM second, community third, and Greenlight falls somewhere further down that list. I look at it when it first came out, and just shrugged and moved on. If an indie game is good enough to break out of Greenlight to more widespread notoriety (such as F.T.L.), then I'll give it a glance. I'm by no means an indie connoisseur, nor do I go out of my way to play indie games for any particular reason. And judging from the numbers, that seems to put me in the overwhelming majority.
Sorry indie developers, but you have to give me a reason to care.