Ignorance.
Terra would not 'erase' Ven rather he would choose sora to serve as a container to store Ventus' body.
Which would explain why the two are observing Sora and Riku sparring on Destiny Islands, Terra would choose Riku as a next in line to inherit the ability to weild the keyblade and Sora as a vessel for Ven's body.
However although Riku was chosen as to be the 'Keyblade master' his heart succumbs to darkness and 'he is not qualified to weild the keyblade', Maleficent grants Riku the ability to weild the darkness.
In Hallow Bastion Xehanort's Heartless possess Riku and delves into the depths of darkness in order to weild a keyblade not of light but of darkness with the ability to release it as opposed to sealing it.
After defeating Xehanort Heartless and Riku, Sora struck himself with the Keyblade of darkness and became a heartless which releases Ven's body however that body did not retain his Heart so instead of Ventus being reborn it gave birth to the nobody Roxas and left Sora to become a heartless however he retained his 'memories' even in this form and returned to his body.
but there is still a problem how can Roxas still exist with Sora's Heart and Body in tact.
the answer Roxas is Ventus' body. Ventus rather than being erased was placed in Sora's body in a sort of 'Sleep' which later gives 'birth' to Roxas.
Thus the phrase 'Birth By Sleep' was coined.
as Master Xehanort foretold Ventus would cross the wall of light and dark as the nobody Roxas.
Quit whoring your damn theory around. I swear, I have seen this exact same paragraph three times and it's starting to get annoying.
Ven asked Aqua to erase him, not Terra.
As far as I know, you cannot have a body within your own body unless you are pregnant.
The titles to the individual games have meaning within their own games but nothing more outside that.
Master Xehanort was talking to Terra in that scene, wasn't he?
Haha I apologize. However you couldn't piece anything together anything huh? not trying to bash your thread but seriously c'mon...Xigbar witnessed the events of birth by sleep as Braig along with the other apprentices of Ansem The Wise, he said it himself. Sora was not meant to weild the keyblade Terra chose Riku thats why Riku could see the keyhole in the Secret Place at Destiny Isle as a child while Sora couldn't. This should be the biggest clue of all and i dunno why No One Else sees it. Remember in Kingdom Hearts 2 where Sora and Roxas encouter one another at the station of serenity in The World That Never Was? Roxas asked Sora "Why did 'HE' choose you?" he meant why did Terra choose Sora as a vessel for his body. then he told Sora "I see, you make a good other." Sora would be Ventus' other existence.
That quote was a mistranslation. It really states, "Why did 'it' choose you?", meaning the keyblade. Not Terra.