They have announced that Warner will be pulling ALL copies of DVD and Blu Rays of Harry Potter at the end of December. The new one comes out November 11th, so grab them while they are out.
Otherwise you will have to wait for the “super special, we jacked up the price and relabeled it the directors cut or the limited platinum gold plated blessed from upon high” edition which will be out eventually.
As mentioned Disney does this, marketing by scarcity. If something is rare people want it more, if its plentiful, well, then its just yesterdays news. Not sure I agree with the philosophy, but I’m sure some one in marketing has a spreadsheet to show it works for Disney.