And I know you meant it lightheartedly, but honestly that is an interesting observation too. I wonder if it's perhaps because mainstream shows like that don't precisely have a fandom in the same sense? It's less subculture and more just culture? Although BBT is the most popular show on TV right now and it still has something of a fandom alongside (if a good deal out of proportion to its size as compared to other works).
Part of it may be just that the fans of the mainstream stuff are less interactive--or may just pay less attention to the tie-in books. In general, I get a lot less mail (as in practically none) about my mainstream books than for the sf/fantasy/horror stuff.
We'll see how things go where LEVERAGE is concerned . . ..