Why Is TV so Low-Quality?

Back when there were only three networks and TV was entirely advertising-supported, this made a lot of sense:

Marginal Revolution: Why is TV so often stupid: Why is TV so often stupid:

TV is not vulgar and prurient and dumb because the people who compose the audience are vulgar and dumb. Television is the way it is simply because people tend to be extremely similar in their vulgar and prurient and dumb interests and wildly different in their refined and aesthetic and noble interests.

That is from David Foster Wallace. Here is the link, from Ben Casnocha.

You see, network TV made money not by providing the maximum value to watchers, but by providing advertisers with the maximum number of eyeballs. The two are not the same.

Now, however, we have cable, and hundreds of channels. So things should be changing. But I see no signs that they are.