The ten tribes of Israel were lost so that later we could realize our purpose with regard to the world. The people of Israel do not exist for their own sake, but to pass on the method of connection and revelation of the Creator to all the nations. All the nations within the common soul of Adam HaRishon, belonging to the body of the spiritual Partzuf, have no way to contact the Creator, except through the Rosh (head) of the Partzuf, that is, the people of Israel, through the two and a half remaining tribes.
The ten tribes disappeared, dissolving into the nations of the world in order to create the connection between the head and the body of the soul. Later, they will be revealed as a transitional link between the head (the people of Israel) and the body (the nations of the world).
Over the past millennia, they have integrated with the body, with all of humanity, but according to their Reshimot, roots, and spiritual genes, they belong to the head. We need a layer in the general Partzuf that includes both the qualities of the head and the qualities of the body.
From the 1st part of the Daily Kabbalah lesson 1/9/20, ”The Jewish Choice: Unity or Anti-Semitism,” Chapter 6
3 Minute 49:07