It's a game of chicken with RBG and the wall.

Trump wants congress to give him the wall so it's done easily/legally. If he calls for a state of emergency, it can be challenged in many courts and would inevitably end up in the Supreme Court.

If RGB stays on the court, she can challenge the legality of it, even to simply delay the construction orders. Right now, he's delaying the announcement to see what Ruthie does with her health/retirement/death.