Why was it so important to be able to live in Israel to the Jews?

Because they believe that God gave them the land. He gave it to their ancestor Abraham for his descendants to live in. See the Bible, Genesis 13:14-17: The LORD said to Abraham after Lot had parted from him, "Lift up your eyes from where you are and look to north and south, east and west. All the land that you see I will give to you and your offspring for ever. I will make your offspring like the dust of the earth, so that if anyone could count the dust, then your offspring could be counted. Go, walk through the length and bredth of the land, for I am giving it to you."