This petition is "Marana tha," the cry of the Spirit and the Bride: "Come, Lord Jesus."
Even if it had not been prescribed to pray for the coming of the kingdom, we would willingly have brought forth this speech, eager to embrace our hope. In indignation the souls of the martyrs under the altar cry out to the Lord: "O Sovereign Lord, holy and true, how long before you judge and avenge our blood on those who dwell upon the earth?" For their retribution is ordained for the end of the world. Indeed as soon as possible, Lord, may your kingdom come!
Source: Catechism of the Catholic Church (Vatican.va)