SpaceX does business with the DoD, which can give them supply-chain priority over everyone else for so-called "DPAS rated" purchase orders. [1] If I'm fulfilling a rated order, I not only have the ability, but the obligation, to inform my suppliers that I'm at the front of the line.

Tesla, presumably, isn't able to play that card.

1: https://www.bis.doc.gov/index.php/other-areas/strategic-indu...