Xemnas Interference
Because: the guy who came up with a back-up plan including time-travel should have, at the very least, have had a plan for what-if's involving his old teacher.

"How are you here?!" DIZ demanded. "Was it that Nobody?!"
Xemnas almost hadn't planned for this.
For one, there was no discernable need. Ansem (the Wise, not his Heartless-self) had been banished to the Realm of Darkness, most assuredly eliminated. In order to leave, he would have to, either meet someone decently powerful with Darkness, or give into Darkness himself. The latter was a laughable theory- his old teacher reaching the enlightenment of Darkness, the worlds would surely become one, before that happened[1]. The chances of one of the few semi-friendly pawns of Darkness coming across him, was just unlikely. He had nothing to bargain with, if he came across the usual Darkness-enhanced individuals (not that there would be many. People didn't just 'visit' the Realm of Darkness unless you were Xehanort. And the old Seeker of Darkness had never came across anyone, except for Maleficent). After the Door to Darkness closed, even that minuscule chance had dwindled drastically. One didn't just travel to the Realm of Darkness, without a lot of work, and Corridor's of Darkness.
As far as Xemnas was concerned, his Master's death was all but proven, certain beyond a doubt. In fact, it could be considered paranoia or insanity, to even consider Ansem's survival, even to Xemnas, however...
He was Xehanort.
Or Xehanort's Nobody, anyway. And a major part of who he was, involved crafting complicated scenario's, that he would likely never use. Planning for every, single divergence life could take (no matter how ridiculous), and how to play it to upmost advantage.
When his Heartless had allied with Maleficent, he had created billions of different possibilities. Including, what he would do if Maleficent suddenly turned to the Light (unlikely, however, if nothing else, his time as a Keyblade Master had taught him not to underestimate the light), to random group betrayal (very likely, if any of Maleficent's assorted organization had actually CARED for Kingdom Hearts, hence they didn't). If Mickey suddenly gave into Darkness (as well as multiple situations where that would be vaguely possibly, and how each reason would affect his behavior), highly unlikely, however, just in case, he was fully prepared. Also, zombie-outbreaks, because it is, actually, a concern in some worlds, and one of the risks of easy traveling, meant that foreign contamination was a possibility, because no one actually wanted to say on a world overrun with undead- it just wasn't practical[2]. He ploted for every eventuality, no matter how unbelievably, and ALWAYS ALWAYS had a backup plan.
One of which, if he failed, included the Sleeping Realm, and time travel.
And, really, how different was Ansem's survival, compared to the random possibility of Maleficent turning her ways.
To this day, Xemnas would say he didn't regret all those hours plotting out all the different details of what Ansem might do. In fact, Xemnas would say, that it was a crowning achievement...
'It made this possible, after all.' Xemnas thought, triumphantly, as he faced his Old Master.
Ansem the Wise lay, on the ground. Looking up, at the Nobody with such hatred, that it stroked the part of him that was still Xehanort's pride. That HE had caused such an indisputably, acknowledgedly GOOD man to turn to Darkness.
"Well, well, Master. It seems that the student has become the teacher." Xemnas said, calmly. The good parts of being a Nobody, otherwise he'd be vibrating at the accomplishment. As it was, he was honestly happier than a Nobody was suppose to be.
"Only the Master of Darkness, my Apprentice." Ansem countered, angrily.
Xemnas laughed. "That would make you, the student of Darkness. Would you consent to an Apprenticeship?"
DIZ as his was current alias, snarled.
"If someone had said that you, Ansem the Wise, would fall to Darkness. I would have labeled them a lair or a false prophet." Xemnas admitted.
"A pleasure, I'm sure." Ansem said, with sarcasm
"If there is one thing to be said for my Heartless, and my past self, is that they were meticulous." Xemnas explained, as if this wasn't out of the ordinary. As if he hadn't doubted the rationality of presuming Ansem could be alive, even theoretically. "Is there any reason, to think I would be any different."
Yes, Xemnas was really, really, glad, he hadn't dismissed the possibility out of hand for being too outlandish. Who knows what his old Master would have done, before he caught on.
"What are you going to do with Sora?"
That, Xemnas didn't know. Yet. "If you had asked me before I discovered your identity, I'd had just eliminated him."
"And now."
"Honestly, I'm reasonably sure anyone could give in to Darkness." Corrupting Sora was not seeming impossible at the moment. Granted, it would be difficult to find the right angle... "There was a reason I never tried to persuade you to Darkness, as I did the others."
Rearranging his world-views were giving such a headache. He really shouldn't be talking to Ansem like this, however it was hard to care about that, and Xehanort was always willing to live dangerously when it came to speeches.
"I'm not perfect, Xehanort."
"Of course not, otherwise we would not have been able to hide so much from you." Xehanort gave a bitter laugh. "However, I had come to expect a certain moral strength."
"Sorry to disappoint, my Apprentice."
"Oh, its not disappointment." Xemnas retorted. "Yes, if I had known so, earlier, I would certain had included you in the Organization."
If Xemnas was reading his Master right, DIZ was experiencing some sort of realization, at being called out so blatantly.
"Well, unfortunately, as it is now. Some member of the Organization are under the particular impression that you are their current leader." Xemnas decided, as it truly was a shame.
It was almost worth the risk, to see Ansem's expressions change from anger and self-hatred, to horror."
"So any Nobody would have to be under your alias." Xemnas continued, as if he hadn't noticed.
"You said they betrayed me!"
"You assumed, when I told you they were accomplices." Xemnas specified. "You didn't specify. Were they involved in the experiments? Yes. Were they knowingly apart of any specific rebellion against you? A couple, yes. All of them? Absolutely not, that would have been...unwise. Ienzo, for one, would have protested, heart or not." And a hand full of others, Xemnas could think of. Despite their differences, they had loved Radiant Garden and Ansem in their own way. Not all of them were as enlightened as Even, willing to make personal sacrifices in the name of science.
"Are you lying, so that I live my last moment in guilt over doubting all my Apprentices?"
"I neither confirm, nor deny." Xemnas stated, smugly. Lips turned upward. For a second, it was almost like old times.
What? He had a right on get revenge on his Master for ruining his expectations. And monologues, those were always fun, and risky (in a way battles weren't when you reached a certain power-level).
DIZ? He had no doubt it was an abbreviation. Darkness To Zero? That sounded like his old Master. Not that the exact wording matter. It was, however, far too long not to be suspicious. Zero wasn't so bad a name, Lea's had been far stranger.
"In any case, you will not be able to find out." Xemnas solemnly declared, using his power over Nothingness to hold the teacher still, while a recently summoned Heartless clawed through his chest.
The Nobody would be a good addition to the cause.
"Welcome, Xezro. The new you."

footnote
1. An old keyberrier saying, that Sora and co. (not including Mickey or Sid) don't know about. Meaning 'when h freezes over', or 'when pigs fly'. Because the aliens from Deep Space, at least, would no idea what H is, or whether pigs have wings or not, and countless worlds would have the same problem. The worlds coming back into one, millennia after they became billions, would seem pretty unimaginable. And it's not like Xehanort would use some variant of 'Kingdom Hearts' because he's trying to make that happen.
2. Or safe. But Xehanort probably wouldn't admit to being creeped out at the possibility of being a brain-dead, disintegrating, monster. Also, seeing Xehanort in a zombie apocalypse of any sort would be interesting. But, he would probably leave, as soon as he was done researching the effects of zombie-fied has on the heart, and be super-careful.
3. Yen Sid certainly wouldn't want to leave all those worlds in the non-physical Realm, if it could be helped. That would be unethical- both by Sora's standard's (it would be wrong to leave them asleep forever), and Yen Sid's (it would be against the natural order).