Why do people think Aqua is the only one who can find Ventus?

[The King of sad]

Seriously why? Although Aqua is the one who transformed Land of Departure into Castle Oblivion, it was Master Eraqus keyblade who actually made it turn into Castle Oblivion. And since it been confirmed that Sora found Eraqus keyblade on the beach of Destiny Islands, Sora can find Ventus himself. All he need to do is sense the connection between the keyblade and Castle Oblivion, go to there, beam a light at the castle, and boom. He finds Ventus. (At first, Sora will think it's Roxas until Mickey tells Sora that's Ventus, and tell him about Aqua and Terra.

 

[Muke]

Because Eraqus said so.

She is the that transformed it, only she knows it's secrets.

 

[Absent]

As revealed through the DDD glossary both Eraqus and Aqua are "Successors" which means they inherit the responsibility of safeguarding the Land of Departure. The Master Defender is tied to that title and responsibility so it stands to reason that only its current wielder can decipher the mystery of Castle Oblivion. After all it was Aqua who reshaped the Land of Departure.
BTW the Destiny Island scene doesn't mean anything, its just a concept trailer so as of now Aqua is still the wielder and Successor.

 

[Xblade13]

Seriously why? Although Aqua is the one who transformed Land of Departure into Castle Oblivion, it was Master Eraqus keyblade who actually made it turn into Castle Oblivion. And since it been confirmed that Sora found Eraqus keyblade on the beach of Destiny Islands, Sora can find Ventus himself. All he need to do is sense the connection between the keyblade and Castle Oblivion, go to there, beam a light at the castle, and boom. He finds Ventus. (At first, Sora will think it's Roxas until Mickey tells Sora that's Ventus, and tell him about Aqua and Terra.

Well, firstly that scene where Sora finds Eraqus' Keyblade? That was just a symbolic scene not a literal one. Aqua still has the Master Defender.

Secondly, as Muke pointed out, Eraqus said that since Aqua, being not only a Master but Eraqus' successor, was the one who sealed Land of Departure, that only she could change it back.

Aqua is the first one of the three that will be saved, as she is needed to get Ven out of Cstle Oblivion and it is likely Xehanort has possession of Terra. That and Aqua is the only one that Mickey has any idea of relating to whereabouts.

 

[Echoecho6]

Wait, that scene isn't actually going to be in the game?

 

[Xblade13]

Wait, that scene isn't actually going to be in the game?

As far as we know, it was a concept trailer not a cutscene. It basically symbolizes that Sora is "picking up the pieces" and moving on to a New journey.

 

[saintfighteraqua]

Wait, that scene isn't actually going to be in the game?

Well, at this point we can't be sure. But I'd say anything shown that early is just promotional footage and won't be included. But you never know, it still could or something similar. It wouldn't really make sense though at this point in the story, in my opinion.

 

[Oracle Spockanort]

Wait, that scene isn't actually going to be in the game?

Probably not, but knowing Nomura he'll find a way to work the idea behind it into the game (i.e. in a dream)

That entire announcement trailer was just proof of concept.

 

[YoungXehanort]

only aqua can if anybody could find ven's room than organization 13 would've found it by now or maybe ven's heart well lead sora to his room

 

[KingdomSlayer2]

Probably not, but knowing Nomura he'll find a way to work the idea behind it into the game (i.e. in a dream)

That entire announcement trailer was just proof of concept.

Tetsuya is the God of concepts. All hail his diddly holiness.
Im trying out the diddly thing.

 

[Anagram]

It literally says so in the game...as She's sealing the castle even.....that very scene.....