Because the Reigns:
Let the heavens rejoice, and let the earth be glad; and let them say among the nations, "The Lord reigns."--1 Chronicles 16:31, NKJV.
THE WORLD that Satan has claimed and has ruled over with cruel tyranny, the Son of God has, by one vast achievement, encircled in His love and connected again with the throne of Jehovah. Cherubim and seraphim, and the unnumbered hosts of all the unfallen worlds, sang anthems of praise to God and the Lamb when this triumph was assured. They rejoiced that the way of salvation had been opened to the fallen race and that the earth would be redeemed from the curse of sin. How much more should those rejoice who are the objects of such amazing love!--Thoughts From the Mount of Blessing, 104.
With eager desire all heaven had waited for the end of His tarrying in a world marred by the curse of sin. The time had now come for the heavenly universe to receive their King....
With joy unutterable, rulers and principalities and powers acknowledge the supremacy of the Prince of life....
Songs of triumph mingle with the music from angel harps, till heaven seems to overflow with joy and praise. Love has conquered. The lost is found. Heaven rings with voices in lofty strains proclaiming, "Blessings, and honor, and glory, and power, be unto Him that sitteth upon the throne, and unto the Lamb forever and ever." Revelation 5:13....
From that scene of heavenly joy, there comes back to us on earth the echo of Christ's own wonderful words, "I ascend to My Father; and to My God and your God." John 20:17. The family of heaven and the family of earth are one. For us our Lord ascended, and for us He lives.--The Desire of Ages, 832-835.