That is perhaps the irony in it. Tom was not concerned with the affairs of men and wizards and elves and hobbits beyond how they pertained to what went on within the boundaties of HIS world. For him to have become part of the quest would have required an interest of a kind which would have rendered him vulnerable to the ring and therefore the very qualities that make him seem to be the ideal candidate would be invalidated were he to become involved. That's just my theory.
-ksen