To begin with and most important is because there is no covenant with anyone accept Ephraim and Judah.
A New Covenant
31Behold, the days come, said the LORD, that I will make a new covenant with the house of Israel, and with the house of Judah:
32Not according to the covenant that I made with their fathers in the day that I took them by the hand to bring them out of the land of Egypt; which my covenant they broke, although I was an husband to them, said the LORD:
33But this shall be the covenant that I will make with the house of Israel; After those days, said the LORD, I will put my law in their inward parts, and write it in their hearts; and will be their God, and they shall be my people.
Besides this, You would make the truth very simple in the last days.
The truth would be this.
God comes to bind them together to fight all nations.
The end of this is all the gentiles bringing all the wealth to the feet of the Jews. I have no problem with that now.
But all the great Messianic prophecies that speak of the Messiah in the last days, is a Messiah that only comes to Ephraim and Judah, everyone else are servants and or dead.
When the spirit is greatly poured out, it is poured on Judah, and Ephraim.
One Nation Under One King
15The word of the Lord came to me:
16Son of man, take a stick of wood and write on it, Belonging to Judah and the Israelites associated with him. Then take another stick of wood, and write on it, Belonging to Joseph (that is, to Ephraim) and all the Israelites associated with him.
17Join them together into one stick so that they will become one in your hand.
18When your people ask you, Wont you tell us what you mean by this?
19say to them, This is what the Sovereign Lord says: I am going to take the stick of Josephwhich is in Ephraims handand of the Israelite tribes associated with him, and join it to Judahs stick. I will make them into a single stick of wood, and they will become one in my hand.
20Hold before their eyes the sticks you have written on
21and say to them, This is what the Sovereign Lord says: I will take the Israelites out of the nations where they have gone. I will gather them from all around and bring them back into their own land.
22I will make them one nation in the land, on the mountains of Israel. There will be one king over all of them and they will never again be two nations or be divided into two kingdoms.
23They will no longer defile themselves with their idols and vile images or with any of their offenses, for I will save them from all their sinful backsliding,
b and I will cleanse them. They will be my people, and I will be their God.
24 My servant David will be king over them, and they will all have one shepherd. They will follow my laws and be careful to keep my decrees.
25They will live in the land I gave to my servant Jacob, the land where your ancestors lived. They and their children and their childrens children will live there forever, and David my servant will be their prince forever.
26I will make a covenant of peace with them; it will be an everlasting covenant. I will establish them and increase their numbers, and I will put my sanctuary among them forever.
27My dwelling place will be with them; I will be their God, and they will be my people.
28Then the nations will know that I the Lord make Israel holy, when my sanctuary is among them forever. 
ALL the prophecies are about Ephraim and Judah.
If you don't fit gentiles in, they have no covenant, and God is not coming to save them and they will find no glory at all.
You are either Ephraim or Judah or you are nobody.