Why must all these things be consummated before Christ's reign is complete?
All these events must be consummated so that Christ can bring to the epitome of understanding—even among believers and the angelic hosts—who He is. The tribulation, Kingdom, Israel's covenant promises, worldwide rule, and the final defeat of death all display His authority and power.
Les explains that after the consummation of the tribulation, the King comes and sets up His Kingdom. Israel will finally enjoy all those covenant promises, including territory clear out to the Euphrates River, and the land will be like the Garden of Eden rather than desert sand. All the earth will come under Christ's rule and reign. After He has shown His authority and power, He must defeat the final enemy, which is death. Les teaches that all these things have to be consummated for Christ to bring to the epitome of understanding—even of all of us as believers and of the angelic hosts—who He is.
For he must reign, till he hath put all enemies under his feet.