Because their paychecks are signed by Treasury. Better not bite the hand that feeds it.
Seems to me they mentioned nationalization during the oral hearing 20 times yet completely ignored it in the opinion write up. Are they afraid of Treasury ?
Asking any Judge for $30B is a big ask (but not when they've stolen $124B!) and the SCOTUS gave zero credence to a very logical and well reasoned answer to the "may be in the best interests of the FHFA" INCIDENTAL POWERS under HERA by J. Brown and the 5th Circuit EnBanc Panel ruling. That the SCOTUS is letting the tail wag the dog here is just wrong. Two well known sayings from historic US Supreme Court decisions say, "Congress doesn't hide elephants in mouse holes" and "You don't hang the muffler on the tail pipe."
But it is whatever the majority opinion at the SCOTUS says it is and that's the way they want it to go.