I think Yoshi-P already gave a pretty good answer about this. It went something along the lines of MS and Sony's new consoles being so new, that they're trying to establish themselves apart from each other (console war, if you will). Right now, it's all about "Experience this content on xbox one only. Experience this content on ps4 only." And so forth. Yoshi-P said that he's thinking that once it dies down a bit and the consoles have had enough time to establish themselves, that MS won't be as reluctant to allow the cross-platform interaction that they're trying to avoid at the moment.