This is an edited version of a theory I posted on KHI, might as well share it here.
Okay, so, after watching the trailer and looking at the name "Birth by Sleep", I've come up with a theory as to why all of these Xehanorts are lurking in the RoL, and why the original Xehanort isn't in the "real world" to begin with. Before I start, however, whoever doesn't know this, Terranort and Apprentice Xehanort should be considered two separate people . This is a key fact, and this is kind of what I'm basing my theory off of.
Let's jump to the BBS Last Episode. Terranort creates a Heartless, the Guardian, and then locks away his heart and becomes Apprentice Xehanort. I'd like to take the moment to emphasize "creates a Heartless". This is important, very important. It is assumed that MX's heart was used to create it, so I'll just call it MX's heartless throughout this theory. Anyways, MX's Heartless was never defeated, only locked away, along with alot of the darkness that came along with it. Which is why Apprentice Xehanort doesn't have golden eyes. Which is why Apprentice Xehanort is a totally separate person from Terranort, because Apprentice Xehanort is essentially Terra while Terranort is MX.
Now, why was all that necessary? Well, when Ansem SoD and Xemnas were created from Apprentice Xehanort's heart (Terra's), and eventually defeated, MX should be able to come back right? WRONG.The Heartless that was a product of his heart was never defeated. And we all know what Nomura said about Heartless'/Nobodies having to be defeated for a person to return. And who exactly is his Nobody? Golden eyed boy? Possibly (won't get into that in this theory though). That's the reason why MX cannot come back.
Now to answer the question "Why are the Xehanorts even in the Realm of Sleep". This has to do with Apprentice Xehanort. AX cannot/will not return because of the fact that MX and Terra basically "split" when AX fell to darkness. Nomura has hinted that Terra's heart is inside Riku. Because MX and Terra split, and Apprentice Xehanort was more or less a fusion between the two, it is impossible for him to return. And so all of the Xehanorts, besides AX/Terranort, are stuck within in the RoS because the original Xehanort is not able to return.
Sound good?