Why were the four angels in Revelation 9 kept waiting?
God kept the four angels waiting because He had appointed a precise time for them to be loosed—a particular hour of a particular day of a particular month. Their loosing was not random; it was governed by God's meticulous timing, showing that He controls the exact moment.
Les explains that the Euphrates River is what he calls a boundary between the eastern religions and those of the west, and these four angels have been confined in that boundary area. In verse 15, these four angels were loosed and were prepared. In other words, God kept them waiting for a particular hour of a particular day of a particular month. Les stresses that God is meticulous in His timing, and he illustrates this by noting that the first temple built by Solomon was destroyed on exactly the same month and day as the later temple that the Babylonians destroyed.
And the four angels were loosed, which were prepared for an hour, and a day, and a month, and a year, for to slay the third part of men.