There is a question whether they should air it, by the end BSG was scrapping the bottom of the barrel on ratings, yes, while I agree with everyone that the old Nielsen ratings are archaic and need to go, that doesn't stop them from existing, and they do and only getting a 1.8 Million demo for such an expensive show to make.. doesn't pay the bills.
BG had problems for two or three reasons, all NBC related.
First they switched to Sunday nights. That really hit them hard as they had developed a following on Friday nights. The ratings were off so much they went back to Friday nights.
Then they did something REALLY stupid. The half seasons followed by 9 month breaks. Viewers have habits. They get into a habit of watching BG on Friday nights. Then after 6 or 7 weeks, it ends. They go watch other shows for 9 months. Now you are HOPING they come back and watch the rest of the season. The word was NBC kept jerking it around. I hated watching the show, getting into it, and then waiting over half a year to watch the second half of the season.
Spartacus was doing something similar but they had the sense to release Gods of the Arena to keep the audience satisfied instead of waiting.