“For behold I am for you, and I will turn to you and you will be tilled and sown, and I will multiply men on you, all the house of Israel, even all of it. And the cities will be inhabited and the waste places built”
The reason that the land would one day again be Israel's was because Yahweh was ‘for them', on their side and acting on their behalf. For He would again turn towards them and be their God. And the land would be possessed again by people from all the twelve tribes (‘all the house of Israel' - and note the stress on this, ‘even all of it'), and would be farmed and well populated. The cities would be rebuilt and inhabited.
That this occurred history demonstrates. While there were Jews who had become disconnected from mainstream Judaism, intermarried or disowning their old nationality as they were absorbed by the nations, or wandering far away and losing contact, members of all twelve tribes were permanently resident in the land up to the time of the destruction of Jerusalem and were among the exiles in Babylon, and others would undoubtedly have returned to it from other places once it began to prosper again. (The concept of ‘Israel' was ever fluid. Still today ‘Israel' is only a small portion of Judaism). And the land would again be populated and prosper, inhabited by those who acknowledged the covenant.
But why was the actual land so important? It was promised to Abraham (Genesis 12:7 and often) to whom the possession of a future land was very important, it was given to his descendants and those who had joined with them in the covenant, and it was essential for the development of a people who would be witnesses to Yahweh as one people. Without the land they would have become divided and fractionalised, and their witness would have been watered down and have disappeared. But once that witness was established and crystallised through the writings of the Old Testament, and the son of David had come, and the Spirit had been poured out, the land ceased to be important and was taken away from them (Luke 21:24). The message had replaced the land.
But the graciousness and faithfulness of God is revealed in that God has not forgotten His promises concerning the land so that it has now, in our day, been returned to those who see themselves as the earthly sons of Abraham. We must not overstress His purpose in this, but His purpose for the future in bringing them back to the land is possibly so that, having brought them together, He can do a great work among them in the eyes of the world by bringing large numbers of them to Christ their Messiah. While Scripture does not specifically require this there are suggestions that this might be so. So we can hope that their receiving again of the land is a preliminary to their finally enjoying the working of the Spirit, as God calls what were once His people to once again become part of His people and accept the Messiah (Romans 11:25-29). The land has become secondary, the blessing of all nations is primary, but this reminds us that God fulfils all His promises, even the secondary ones.