Well, we can assume that Terra was on the islands choosing Riku to be the next Keyblade wielder, and Ven has his obvious connection to Sora, so that leaves Aqua as the one who has no connection to SRK.
Now lets say Ven did chose sora to wield
That would imply that Ven has the authority to choose the next wielder, which he does not. Terra and Aqua are the only ones taking the Master Qualification Exam, and apprentices can't choose future apprentices (since they obviously were not expecting to be wiped out).
the Keyblade goes to Sora because of the connection between Terra and Ven.
The connection is called "brotherly friendship." And like what everyone else said, the Keyblade chooses the one with the strongest heart. Since Riku embraced the darkness, the Keyblade chose the next strongest heart, which happened to be Sora.
So once a replica dies the memories of it will die as well As if they never existed.
That's not the case with Repliku.
Because of Ven's memories hidden away deep inside Sora and because of the short time he had as a heartless Roxas didn't have time to gain memories of his past life so took the immediate shape of Ven.
Ven being sealed within a random 4 year old? Since nobody (< see lowercase n) was expecting Riku to lose himself to the darkness, there's the chance that Sora was completely overlooked.
Like when Kairi forgets Sora and Riku but still remembers them and that they used to play together.
Kairi remembered Riku perfectly. It was Sora who's memories of were forgotten.
Originally Posted by Ruran
I got the feeling that sh e was probably just trying to buy Cinderella some time while she tried to get down stairs or something.
I think that too, since something had to make her gasp like that. Also, because she was lower to the ground, that means there's a higher possibility of her seeing Ven and Jaq with the key, so she stalls for time by asking to try on the slipper. But I'm getting a feeling that the slipper will fit her...