I know why. Because if a 2 letters .org is deleted, they are no longer available to register. They will be reserved at registry. Why I know? Because I have my PHD in domain studies.
2-char .org may no longer be registered even though they show up as free in your whois queries. Dropping 2-char .org will be redeemed by the PIR registry upon expiry so it looks like a rare breed on its way to extinction ;-)