God has always used man to preach to the masses, not angels (who seem to only appear to the mature righteous). The 3 angels with their messages to preach are introduced when the 144,000 are shown after their sealing with protection on Mt Zion. This explains the reason for their sealing, they have work to do in a hostile environment: fulfill Mt 24:14 which has never been done before.
Matthew 24:14 — And this gospel of the kingdom shall be preached in all the world for a witness unto all nations; and then shall the end come.
See, the world has chiefly heard only a little about Jesus, but has not heard the full and accurate message of the Kingdom of God (including what it is exactly, why it is not here now, when it is coming, and most of all, how to qualify to enter it). So the angels are delivering this lost message to the 144,000 to preach. It does not say the angels themselves preach it if you read carefully. It would be too convincing and negatively affect our free will if angels were preaching the gospel.
But notice also that the purpose of the preaching in Mt 24:14 is not to convince everyone, but to serve as a witness before judgment can come. In this way everyone who wanted to repent had the information to do so and could have done so and thus could not say they didnt know or that God is unfair to judge the earth as harshly as he will.
This underlines the fact once again that God is not trying to save everyone now. He is calling his Firstfruits who have the tenacity to choose his way repeatedly against the opposition of Satan and his minions, the world, and their own nature. According to the Bible, their reward is to reign as kings and priests for 1000 years on earth before the Great Harvest "of the rest of the dead" is bought in.