Before David was chosen (as king) every Israelite was kasher for kingship. Once David had been chosen, the other Israelites (i.e., those not in his line) were excluded. As it is written (II Chronicles 13:5) "Is it not for you to know that the L–rd, the G–d of Israel, has given over the kingdom to David, to him and to his sons?" If you would contend: But the L–rd did speak with the prophets outside the land, (I would answer:) Though He spoke with them outside the land, He did so only in the merit of the fathers. As it is written (Jeremiah 31:15-17) "Thus said the L–rd: A voice is heard in Ramah … Thus said the L–rd: Keep your voice from weeping, and all your eyes from tears … And there is hope for your future, says the L–rd, etc."