The events described in Mathew 24:15-20 (A) took place during the Jewish wars with the Romans in 70AD, which was God's Judgment. The events described in Matthew 24:36-40 (B), which is yet in our future, are not a judgment of God; they are a test.
(A) Judgment ------> Jerusalem
(B) Test of faith <----- Jerusalem
Think of it this way, Both (A) and (B) can be understood from the perspective of two moments of escape: (A) is the escape FEOM Jerusalem, and (B) is the escape TO Jerusalem.
(A) Judgment
In this case, Jesus warns the people to flee Jerusalem and head into the mountains because, as Daniel says, the temple and the city will be destroyed and come to utter ruin. People are not to get their stuff from the house or come in from the fields. They should pray that their flight not be on a Sabbath.
(B) Test of faith
I call this a test of faith because that is how it is pictured in the prophets Malachi and Joel. Both of these prophets talk about fire and burning, when the entire land will be destroyed by fire. In this situation, the people are to flee their own homes in haste to travel to Jerusalem for safety. Just as in situation (A), people are not to get their stuff from the house or come in from the fields. They escape destruction by traveling to Jerusalem as fast as they can. (Isaiah also talks about fires and survivors.)
Apparently, this is the point at which Israel's enemies come to Jerusalem to wipe out the Jews once and for all. And this is when God protects her and when Jesus returns.
(A) Judgment
(B) Test of faith
I just wonder on what basis does prophecy get divided up like this >>> already happened, still needs to happen?
Let's look at Joel,
And everyone who calls on the name of the LORD will be saved; for on Mount Zion and in Jerusalem there will be deliverance, as the LORD has said, even among the survivors whom the LORD calls.
This is not an instruction to flee there rather that there is deliverance there, there were survivors who survived the destruction in 70 AD.
Jesus tells the first century believers to flee from Jerusalem and they did but there were some who did not.
We can know this because Peter quoted from Joel on his sermon on Pentecost which means it was directly applicable to the times that were coming soon.
Jerusalem also burned in 70 AD.