Some things that probably helped Birmingham is its proximity to Atlanta and Nashville (reuse the same spots, close to major IP fiber networks, etc) And also the fact that Fox 6 is owned by News Corp.
I live in birmingham and its not that great an area to get HD reception. The cable companies (Bright house in B'ham proper, Charter in the suburbs) don't have all the networks yet, and if you live further than about 15-20 miles outside of the city then you can't get them OTA very well. Also, the ABC station is split between Tuscaloosa and Anniston, and there is a very well known problem getting these stations, even in analog, OTA.
And while the city yes is losing population, the metro area has a very large population, but the DMA includes counties well outside the metro area which is very very far away from birmingham, meaning that HD OTA reception in some parts of the DMA is impossible.