...but isn't the income from sheet music sales pretty much zero when the music is for a television series nobody has seen (i.e. a failed pilot)?
Well, you never know. A failed pilot, maybe not. But a short-lived series? I seem to recall my father having a couple of musicians' "fake books" that included the themes to one or two relatively short-lived shows. Sometimes the popularity of a piece of music will have more to do with the music itself than the show it was attached to.