"because I say so" invokes the very fact of the demand as the supreme reason, rather than acting as a promissory note, saying "no time to explain now, but trust me there's a good reason that I'll explain later"

ie, "because I said so" is "bow to my authority, underling" rather than "in this specific circumstance, just do it, trust me (for now) there's a reason, and ask later if it's not obvious to you by then"