Because the city and sanctuary were already desolate before their end( consummation).
The sanctuary where the Holy of Holies was, was made desolate after Christ's crucifixion, when the outer vail was torn in two.
The sacrifices related to the sanctuary which were ended right there and then, after Jesus said it is finished.
The City of Jerusalem itself, was consumed to its end, along with its citizens, who were a desolate house (sanctuary), without the bridegroom Jesus Christ.
What is the mystery of God?
A mystery is something that is kept until the final consummation of the bride of Christ.
the mystery of God would be fulfilled, just as he announced to his servants the prophets.
You see this mystery was prophetically revealed, but had been sealed from being made known, until the end. When Jesus said, this gospel of the kingdom will be preached in the whole world as a testimony to all nations, and then the end will come, he pointed to his brilliant coming at the end of the New Covenant Age, where the Great Commission would have run its course and made the completed list in the book of life, from before the creation of the world.
The abomination that maketh desolate that appears in the Holy Place is future, within the body of Christ, the temple that is not procured by human hands.
This is important, because the mystery of God is revealed, after the mystery of lawlessness is exposed.
I disagree with Full Preterists that this happened in 70AD or any time in history.
However recently, I have observed that the abomination is an act to make Holy something that desecrates the inward temple of God, by a lifestyle that has become prevalent within today's society, that is no longer a secret behind the secret confines of closets, but is now openly celebrated in the public, just like Sodom was doing during Lot's time.
The abomination of desolation comes before the mystery of God is revealed, when Jesus overthrows the Man (Anthropos/People) of Sin with the breath of his mouth (through his true body of believers) and destroys by the brightness of his coming (7th Trumpet).
which was not made known to the sons of men in other generations as it has now been revealed to his holy apostles and prophets by the Spirit. This mystery is that the Gentiles are fellow heirs, members of the same body, and partakers of the promise in Christ Jesus through the gospel.
That is true, for it is one of many mysteries revealed to the Apostles in the last days of the Old Covenant Age, but it was NOT the Mystery of God as the definite article, pointing to his Appearing on the Last Day of Judgement.
26Even the mystery which hath been hid from ages and from generations, but now is made manifest to his saints:
27To whom God would make known what
is the riches of the glory of this mystery among the Gentiles; which is Christ in you, the hope of glory:
28Whom we preach, warning every man, and
teaching every man in all wisdom; that we may present every man perfect in Christ Jesus:
29Whereunto I also labour,
striving according to his working, which worketh in me mightily.
So, you are partly right, that the mystery that was revealed to the Old Testament Prophets, was unveiled to the Apostles the Saints, whereby the mystery is an ongoing work in progress, until it is finalised at Christ's appearing, where the 7th Trumpet is blown and time no longer declared.
So in this respect the message or mystery of the good news doesn't begin and end in the 1st Century, but it is a slow reveal to every generation, until the end trumpet is sounded. So in the sense of the mystery of God being proclaimed, it is a work in progress, so long as disciples are being made of the world and then Jesus Christ calls it a day, when he sounds the 7th Trumpet.
By then everyone would have gotten the good news and that God will be all in all who were written in the book of life, at the culmination of the New Covenant Age, at Christ's appearing. Then Christ would have defeated the final enemy biological death and spiritual death in Hell and would become subject to the Father, indicating that his Cross of Salvation would have run its course.