The looming court seat was a very effective driver of votes for Trump in the previous election, at least that's the conventional wisdom. So have a go at seeing if that will work again. You can also spin it as being respectful, serious, and considered by waiting.
Even if you end up losing the Presidency, and even the Senate, you're still in power for enough time afterward to make an appointment.
The only things I could think of working against that would be: the idea that you just make the appointment immediately and tout it as another success to your base, or perhaps a fear that since it's Ginsburg's seat that's vacant, having it open is actually a more mobilizing force for Democrats. There's also, I suppose, the possibility that you make the move right away on the theory that this upcoming election is likely to be contested and could reach the Supreme Court.
The risk of not winning either is too much. They'd rather get the "win" and campaign on the win.
But this is also exactly why this gambit doesn't really work - because the voters know that whatever happens at the polls, the end result is the same. They could hold voters hostage to the Supreme Court nomination back in 2016 because there was a split between the president and the Senate that blocked either side from getting what they really wanted until the next term.