Most people don't make a habit of watching a bunch of news from a network that leans in a different direction, so most people don't have the opportunity to realize what they are not being told.
I agree with MAC: the problem lies with those who are lazy and choose to passively consume what they're fed - as the Founders held, the price of liberty is eternal vigilance, and these folk aren't paying the bill - and the fact of their laziness says nothing about how the so-called "Big 6" operate.
Furthermore, I find it rather odd that there is this presumption that having just 6 separately owned media sources is somehow evidence of a cabal, a cartel, or a conspiracy. After all, there are really only 7 separately owned auto manufacturers whose products one can purchase in the US: Ford, GM, Chrysler (really should be called Fiat by now), Hyundai, Nissan, Toyota, and Honda - there are some others, e.g., Mitsubishi, but they have very little market share (they're sort of the equivalent of nonconventional, online info sources).
Since there are only 7, are we to conclude that this is evidence of collusion amongst them to restrict the cars that Americans can buy? Puh-leez. And to see why it's absurd to blame the small number of media outlet owners for the fact that many individuals only get their news from one source and so have an unbalanced view - which carries with it the necessary implication that those sources should be forced by government diktat to provide "balanced" programming (assuming one can define "balanced") - consider the number of people who have strong brand loyalty and only buy, say, Fords, without ever considering any of the other manufacturers. It's not unreasonable to say that when you're shopping for a new car you should look at everything that's available - compare and contrast and go with the best, regardless of brand - but the fact is that many don't.
If someone like that ends up buying a lemon because they didn't consider all the alternatives - say they buy a GM only to discover later on that the car they bought has a tendency to burn out the tranny early, a fact that they would have learned about if they'd done their research - whose fault is that? Is it the fault of the auto manufacturers because there are only 7 of them and because the person in question bought based on brand loyalty and not on research?
I don't think so. If someone who buys a car solely because of brand loyalty buys a known lemon, that's his/her fault; just so, if someone gets an unbalanced view of the world solely because he/she chooses to watch only one outlet, that's his/her fault, not the fault of the media industry as a whole.