You obviously don't understand politics. How bad would it look if he kept the same date/time for his address as the GOP? That would look like dirty politics and hurt him further in his bid for reelection. Republicans would have a field day with that!
You have a severe lack of understanding of the U.S. Constitution.
Merabers of Congress do not work for the President, they are merabers of the Legislative branch of government, a co-equal branch of government as the Executive and the Judiciary.