I can't find a source for this info at the moment, but I believe Gwynne Shotwell saying that they'd actually approached NASA (and possibly other customers) and

I can't find a source for this info at the moment, but I believe Gwynne Shotwell saying that they'd actually approached NASA (and possibly other customers) and asked if they'd like to fly anything on the Falcon Heavy demo mission. Clearly, nobody took them up on the offer.