First of all, and I'll give only one or two examples of why, with the exception of the destruction of Jerusalem which was fulfilled in 70 AD, the events detailed in Matthew 24 have never been fulfilled but will be in the future.
In verse 15, it says the period of intense persecution of Jews will begin when "the abomination of desolation," spoken of by Daniel, is seen "standing in the holy place." We have no historical record of such an event taking place in 70 A.D. Unlike Antiochus Epiphanes, who desecrated the Temple's holy place in 168 B.C. by erecting within it an altar to Zeus, Titus took no such action in 70 A.D. before he destroyed the city and the Temple.
Jesus says that the period of "great tribulation" (verse 21) that He is talking about will be so severe that all life will cease unless the period is cut short.
We live in the only generation in history when these words could be literally fulfilled. There was no possibility in 70 A.D. that the siege of Jerusalem would lead to the extinction of all life. But that is a very real threat today due to the development and deployment of nuclear weapons.
In verses Jesus says that all the things He has spoken of concerning the Tribulation will be fulfilled during the generation that sees the "fig tree" reblossom. Here is the key to the timing of the prophecy's fulfillment.
What is the "fig tree"? What happened the day before? Jesus had put a curse on a barren fig tree (Matthew 21:18-19), causing it to wither. It was a prophetic sign that God would set the Jewish nation aside because of their spiritual barrenness — that is, their refusal to accept Jesus as their Messiah. The fig tree is a symbol of the nation of Israel (Hosea 9:10; Jeremiah 24:1-10; Joel 1:7; Luke 13:6-9).
Now, the next day, Jesus calls the fig tree to mind and says, "Watch it. When it reblossoms, all these things will happen."
The setting aside of Israel occurred in 70 A.D. The reblossoming took place in 1948 when the nation of Israel was re-established.
Matthew 24 has yet to happen. It is going to be fulfilled soon; Israel has been regathered, the nation has been re-established, and the nations of the world are coming together against the Jewish state. The wrath of God is about to fall. We are on the threshold of the Great Tribulation. And the Great Tribulation has never happened in all of human history.
BTW Did You catch the fact that when I say replacement theology is a lie I dropped the IMHO yesterday? Replacement theology is as insidious as idol worship.