A lot of people don't get particular networks because the US recently (June 2009 [1]) switched to digital TV broadcasting. Essentially, digital TV is a transmission protocol which more efficiently utilizes the available electromagnetic spectrum by transmitting compressed video.
The problem is that digital transmission doesn't gracefully degrade like analog transmission does. If you're at the outer part of a transmitter's range, with the legacy analog signal, you might have been able to get an adequate signal with slightly fuzzy picture or static-y sound. But receiving a digital signal is largely a binary affair; you're either in-range and receive it, or out-of-range and you don't. *
Another issue is the economics; most people get their TV through cable. So it doesn't necessarily make economic sense for stations to build new transmitters to make their post-digital range equal their pre-digital range, especially if the new dead zones are in low-population-density areas.
Online streaming is so technically superior to broadcast -- there's no technical reason you can't get what you want, when you want it; and the maximum number of channels we can support isn't limited by scarce electromagnetic spectrum -- that I foresee traditional broadcast TV being completely replaced within the next 10-30 years.
It's still new enough, however, that the social, political, and legal issues still have to be worked out. I.e. FCC requirements to provide broadcast-equivalent service should probably apply to ISP's, but they don't. And companies like NBC should realize it's in their best interest to make content available at reasonable prices "ala carte" online, but they don't.
It's particularly interesting to see that recently services like Hulu or Youtube are running pilot programs for producing original content. Essentially the reason NBC et al can get away with offering customers awful service without having their lunch eaten by startups is that they by-and-large have a monopoly on content that people want to see. But if similar content becomes available from other services at lower cost and/or on saner terms, they'll be forced to change their business or die off. In other words, someone needs to do to NBC what iTunes did to the music industry. There are plenty of people in this space; Hulu, Netflix, Youtube, many others.
Online video is still a very immature industry because only within the last 10-15 years has bandwidth and decoding horsepower sufficient for streaming video become available to most people. Over time the free market should iron out a more efficient solution to connecting viewers, content producers and content aggregators, but many existing businesses are enormously large and entrenched, so the process will take time.
[1] http://en.wikipedia.org/wiki/Digital_terrestrial_television#...
* This isn't strictly true; you might get some artifacts in marginal situations, but these tend to be much more noticeable and much less acceptable than with analog. The "in-range" and "out-of-range" might change over time due to weather.