It changes the circumstances of our experiments. It changes the conditions of the promise I made to you. Giving a keyblade to someone who can inherit his own? While we can at least be sure you'll be able to wield it, in turn, you're no longer a blank slate to work with. And equally, if you were to inherit before that point, it would make the outcome unclear.

At the same time, I did make you that promise.

[One of those rare odd phrasings on his part where it's unclear whether the 'I' refers to his present or past incarnation.]