Why can much happen in the 75 days after the tribulation?
Because 75 days is a long period for God to operate. During that interval He will fulfill Daniel 2 by striking the image of Gentile empires at its then-functioning iron-and-clay stage, the revived Roman Empire, and crush everything connected with Gentile rule before the Kingdom proceeds.
Les explains that once the church age believers and the first fruits have been taken out, and now the Old Testament believers and tribulation believers are resurrected, all believers of the age are complete, and the Kingdom is ready. But since the resurrection of the Old Testament saints is 75 days into the Kingdom, there has to be a reason for that interval. Les points back to Daniel chapter 2 as the explanation. For God, of course, a lot can happen in 75 days; it is a long time for Him to operate. In Daniel 2, Nebuchadnezzar’s dream concerns the great empires of the world pictured in the image—the head of gold, chest of silver, belly of brass, legs of iron, and feet of iron and clay—all pictures of the Gentile empires coming down through history. When the Lord returns at the end of the tribulation, He will strike that image on the empire then functioning, the iron and clay, or the revived Roman Empire, and He will crush everything that smacks of Gentile rule and empire.