I saw someone else explain this, I will try to paraphrase and get the bulk of it so bare with me. Microsoft doesn't put games or expansions ( dlc ) in its stores until the game is ready to be played and is done being worked on. While PlayStation works differently and puts it up earlier ( I forgot the reason why ). But since Rise of Iron is still being worked on , it isn't in the Microsoft store. Once it is done they will put it up for pre order. I have also seen one or two replies from bungie about this saying don't worry it will be up for pre order with plenty of time ton spare before it comes out. My guess is it will be end of August or beginning of September.