Why do the angels say 'now is come the kingdom of our God' in Revelation 12:10?
Because Satan, the accuser of the brethren, has been cast out of heaven after Michael and the holy angels defeat him. His final access to the throne room ends, so the angelic host announces salvation, strength, the kingdom of God, and the power of His Christ as God's earthly kingdom draws near.
Les explains that Satan had come from going to and fro in the earth, his domain where he rules and reigns. In Revelation 12, Satan has now had his final access into the throne room, or heaven, and as a result of the war in which Michael and the holy angels fought and defeated him, he is cast out. Then verse 10 declares, 'now is come salvation, and strength, and the kingdom of our God, and the power of His Christ,' because the accuser of our brethren is cast down, who accused them before God day and night. Les points out that Satan is the accuser, accusing believers every time they sin, constantly letting the Lord know where we have failed. That kingdom has been in view ever since the beginning of human experience—the glorious kingdom God will rule and reign upon the earth—and now it is beginning to get close, so the angelic host can make this announcement.
And I heard a loud voice saying in heaven, Now is come salvation, and strength, and the kingdom of our God, and the power of his Christ: for the accuser of our brethren is cast down, which accused them before our God day and night.