It’s way too early for the candidates to commit to these decisions. First, it’s too early to know who will be able for Cabinet positions. We wouldn’t want a candidate to name someone of lesser quality because a better staffer isn’t yet available. It would also be embarrassing for a candidate to name someone now, who then has to announce they won’t be taking the position because their personal circumstances have changed.
On a more practical and unpleasant level, there are still a lot of deals that have to be cut, and the Washington elite don’t want to commit to a position in a candidate’s Cabinet, only to have that candidate not get the nomination, and the rival candidate has filled all of their spots. The candidates also don’t want to commit a spot away now when they might be able to cut a better deal later, say, you throw your support behind me in this state’s primary and you’ll be Secretary of Energy.