Those ratings are based on percentage of of people in a particular area watching TV, not absolute viewership. The greater Birmingham metro area has a population of just over 1 million. The greater Miami metro area has a population of 6.6 million. Birmingham has a Nielsen # around 9. That means 9% of households are watching Bama football, which equates to roughly 100,000. Bama is as dominant as it can be so there isn't much else they can do that is going to suddenly get more people in Birmingham to watch Bama football. Miami could have an absolutely abysmal Nielsen rating of 2 and would still have more total viewers (120k+). In addition to that, Birmingham is ranked amongst the poorest cities in the U.S, meaning national advertisers are fully aware that people there aren't buying a lot of products and it's a bad market (shouldn't come as a surprise that advertisers don't advertise for the sake of advertising, they do it because they want people to buy their stuff).
Which market do you think national advertisers are going to be more interested in- poor, decrepit Birmingham where there is nothing to do but watch football or the capital of Latin America, which has 6 times the number of potential viewers and where everyone feels like they need to have the latest and greatest products? And if Miami actually has a good season and more people in S Fl start watching games, it will obliterate Bama's local TV numbers.