Two reasons: 1. With system files, file locking, the registry, and similar complications, Windows makes backup software far more complicated than necessary -- in this case, "expensive" means "it would take a long time to write all the necessary code". 2. Windows users are generally less clueful, and therefore require more time-consuming hand-holding.

EDIT: There's also another benefit to starting with FreeBSD: Many FreeBSD users know and trust the project's security officer, while most Windows users haven't a clue who I am. :-)