Hi myst33,
Because the content of Matthew 24:15-51 is time of the end. The reference Jesus made about the abomination of desolation of verse 15 was to what Daniel the prophet was told to express.
The abomination of desolation is found in Daniel 12:11-12, and Daniel 12 starts off with the worst time in Israel's existence (the great tribulation in Matthew 24:21) and in verse 4 is time of the end.
4 But thou, O Daniel, shut up the words, and seal the book, even to the time of the end: many shall run to and fro, and knowledge shall be increased.
There are other important time of the end passages to happen during that same time. The vision that Daniel had in Daniel 8 about the little horn person and the transgression of desolation are said to be time of the end as well.
16 And I heard a man's voice between the banks of Ulai, which called, and said, Gabriel, make this man to understand the vision.
17 So he came near where I stood: and when he came, I was afraid, and fell upon my face: but he said unto me, Understand, O son of man: for at the time of the end shall be the vision.
The Antichrist (the little horn) going into the temple, sitting, claiming to have achieved God-hood is the transgression of desolation.
Later, after he is killed for his act, and brought back to life as the beast, a statue image of him will be made and placed in the temple courtyard, as the abomination of desolation.
Which Jesus told them in Judea to flee to the mountains when they see it.
Matthew 24
4 And Jesus answered and said unto them, Take heed that no man deceive you.
5 For many shall come in my name, saying, I am Christ; and shall deceive many.
11 And many false prophets shall rise, and shall deceive many.
You agree that the above were fulfilled before 70 AD.
23 Then if any man shall say unto you, Lo, here is Christ, or there; believe it not.
24 For there shall arise false Christs, and false prophets, and shall shew great signs and wonders; insomuch that, if it were possible, they shall deceive the very elect.
25 Behold,
I have told you before.
26 Wherefore if they shall say unto you, Behold, he is in the desert; go not forth: behold, he is in the secret chambers; believe it not.
In verse 25, Jesus links His words in verses 23-26 to His words in verses 4-5, 11.
The "you" in verses 23, 25, 26 is the same "you" as in verse 4. Jesus' audience does not change.
Therefore verses 23-26 are
not about the "time of the end", but
also about the period before 70 AD.
Historical confirmation:
Ralph Woodrow, Great Prophecies of the Bible
Did such deceivers or false Christs arise and deceive many in those years before the destruction of Jerusalem? Yes.
According to Josephus, the noted Jewish historian, twelve years after our Saviour's death, a certain impostor named Theudas persuaded a great multitude to follow him to the river Jordan which he claimed would divide for their passage. At the time of Felix (who is mentioned in the book of Acts), the country of the Jews was filled with impostors who Felix had put to death EVERY DAY—a statement which indicates that there were "many" of such in those days.
An Egyptian who "pretended to be a prophet" gathered 30,000 men, claiming that he would show "how, at his command, the walls of Jerusalem would fall down."
Another deceiver was Simon, a sorcerer, who led people to believe he was the great power of God (See Acts 8). According to Irenaeus, Simon claimed to be the Son of God and creator of angels. Jerome says that he claimed to be the Word of God, the Almighty. Justin relates that he went to Rome and was acclaimed as a god by his magical powers.
Origen mentions a certain wonder-worker, Dositheus, who claimed he was the Christ foretold by Moses. Another deceiver in those days was Barchochebas who, according to Jerome, claimed to vomit flames. Bar-jesus is mentioned in
Acts 13:6 as a sorcerer and false prophet.
These are examples of the deceivers of whom history says there were "a great number," and of whom Jesus had prophesied that there would be "many."