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.

When pre ordering off xbox one you gotta download something but there is nothing to download at moment so cant pre order bungie said on twitter 4 days before release day you can pre order on xbox one and download file but obviously cant play till 4 days later which means there still tweaking bits here n there as there is no download file available yet

Never !!! No iron gjallarhorn for you :(

I'm fairly sure it was a ruse to allow the black ghorn to be a PlayStation exclusive without Xbox players having a conniption about it. I'm an Xbox player and you can bet I'm pissed

It recently changed to bundle only from coming soon. So think it will be up within a couple of weeks!

Please pay attention to the thousands of other posts..soon. bungie has said soon. I think there's a problem with Microsoft lately