XD Well, it's implied that you get your X being in the Org. "Their names all share something as a mark of their brotherhood", or something, I believe was the quote.
That, and since they changed their name (presumably) when they got Roxas, it's possible that when Roxas turned up, Xemnas figured the Organization was complete. Okay, sure, we'll take in that other Nobody too, but Roxas is the Organization's capstone. Further members are superfluous.
no subject
Okay, I see it like this. You get two teenaged Nobodies at the same time. One of them is in great physical shape, and turns out to be the Nobody of the Keyblade Master and can, consequently, kick absolutely ridiculous amounts of ass. The other is a wispy little girl who likes to draw. Which one are you gonna want more as a member? XD The Org does seem to value the ability to kick some ass when threatened - Zexion doesn't fight, sure, but that's personal preference and he's one of the original six anyway. Naminé was useful, but probably wasn't judged capable of doing Org stuff.
That, and since they changed their name (presumably) when they got Roxas, it's possible that when Roxas turned up, Xemnas figured the Organization was complete. Okay, sure, we'll take in that other Nobody too, but Roxas is the Organization's capstone. Further members are superfluous.
... S'my guess, anyway.