Why were the branches of Israel broken off from the olive tree?

Les explains that Israel, the natural branches, were broken off because of unbelief. They had been in a place of blessing, enjoying the fatness of the root of Abraham—that is, the promises—and they had the Word of God and everything else going for them. But they rejected it in unbelief and lost that exalted position. The Gentiles, a wild olive tree, were grafted in and now partake of the root and fatness, experiencing that position for almost 2,000 years. Yet the context notes the Gentiles have also walked it underfoot and cast it aside, saying, thanks, but no thanks. The reason for Israel's breaking off remains unbelief.