Why would he write about the destruction of Jerusalem? He was writing about "what is now" and "what is to come".
Revelation 1:19 Write the things which thou hast seen, and the things which are, and the things which shall be hereafter;
Look at the "churches" he was told to write to. No "church" in Jerusalem? Jesus was sent to the Jews. He didn't address them at all? The only explanation is that they had already been dispersed and the Temple destroyed.
Acts 8:1
And Saul was consenting unto his death. And at that time there was a great persecution against the church which was at Jerusalem; and they were all scattered abroad throughout the regions of Judaea and Samaria, except the apostles.
The church was scattered in about 42 AD.