The judgment of God traps us so that we cannot escape. The fall of Jerusalem to the Romans in AD 70 was still forty years away, but nothing could prevent it coming.
God has many ways of trapping men in his judgment. He can take them by surprise so that they do not expect what he is going to do. Or he can tell them what he is going to do, as here, and they still cannot escape, because they do not take his word seriously, and so they do not act on his warning. But he says this for men on earth to hear, before it happens, so that some individuals might be moved to repentance. Some escaped the awful events of the Roman siege of the city, and these were forewarned in advance by Christ what they should do to escape (Mattew 24:16).