It seems like, no matter what TV show I'm looking at in the iTunes store, it says "Viewers Also Bought: Battlestar Galactica".
Ain't nobody here but us turkeys [youtube.com]
Amazon admitted once that they sometimes get paid to provide such messages to the users... maybe iTunes does the same. Or BG is really popular.
I think it's also because most of the people buying TV shows off of iTunes are more likely to geeky early adopters.
The percentage of these people who have watched BG is >>> than the percentage of average Americans who have.
On a related note, it looks like the money-men consider BSG to be the height of technology:
What is a dice and how does one roll it? [arstechnica.com]