The three big reasons are that 1/2 of the people are at any time on the slower side of the curve. There exist some quality we call intelligence and half of the people are below average by definition. Furthermore, at any time most people are simply trying to survive and don't want to think about cosmology. Last for the same old reasons as in the past, in today's America, hearing this content from an African American is just too much for some people. It does not fit the racial profile. Society is evolving and will get beyond that eventually. We are not there yet.
Personally, I think the show is doing a really good job of explaining the topics of cosmology, biology, and astrophysics. This production has woven science into a human story of questions and discovery. A story which anyone with the willingness and capacity to listen to will learn from. Particularly in the science focused media. We tend to think that the general public is very interested in the mysteries of the universe.
I defy anyone who would dispute my points about the country's readiness for this message and the messenger to point out an objective flaw in the presentation above that would explain why relatively mindless entertainment always does better than it.
Cosmos A Spacetime Odyssey will be remembered in ten, twenty or thirty more years as a great documentary which will have placed our current understandings of science in front of the general public. This will be remembered and appreciated by a future world much more than it is now.
The impact of a show like Cosmos will be if it inspires just one or two people who are now children to love science
Updated 4/13/2014 to add: The reason Cosmos's ratings pale in comparison to inspiring even one young person to seriously study science comes down to the impact that a scientist may have on those who come after them. To see that all you have to do is imagine a world without Marconi and his radio, much less a world without a Newton. Cosmos, like most works of science will not be a big fast money maker, it is a play for the long haul.