And we know this passage is speaking of the judgment of Jerusalem because it had happened before:
Ezekiel 5:8–9 (NKJV)
8 therefore thus says the Lord GOD: ‘Indeed I, even I, am against you and will execute judgments in your midst in the sight of the nations. 9 And I will do among you what I have never done, and the like of which I will never do again, because of all your abominations.
The instrument of God's judgement then was Babylon, led by Nebuchadnezzar. Jerusalem and its temple were destroyed in 586 BC.
Why did God do this?
2 Chronicles 36:15–17 (NKJV)
15 And the LORD God of their fathers
sent warnings to them by His messengers, rising up early and sending them, because He had compassion on His people and on His dwelling place. 16
But they mocked the messengers of God, despised His words, and scoffed at His prophets, until the wrath of the LORD arose against His people, till there was no remedy.
17
Therefore He brought against them the king of the Chaldeans, who killed their young men with the sword in the house of their sanctuary, and had no compassion on young man or virgin, on the aged or the weak; He gave them all into his hand.
Matthew 24:21 begins with a conjunction "for then" (because), which means it's referring to an earlier statement or idea. In this case, we know what that earlier idea is:
Matthew 23:31–36 (NKJV)
31 “Therefore you are witnesses against yourselves that you are sons of those who murdered the prophets. 32 Fill up, then, the measure of your fathers’ guilt. 33 Serpents, brood of vipers! How can you escape the condemnation of hell? 34 Therefore, indeed, I send you prophets, wise men, and scribes: some of them you will kill and crucify, and some of them you will scourge in your synagogues and persecute from city to city, 35 that on you may come all the righteous blood shed on the earth, from the blood of righteous Abel to the blood of Zechariah, son of Berechiah, whom you murdered between the temple and the altar. 36 Assuredly, I say to you, all these things will come upon this generation.
And he repeats this idea of their persecution to His disciples when discussing the signs for the end of the age:
Matthew 24:9 (NKJV)
9 “Then they will deliver you up to tribulation and kill you, and you will be hated by all nations for My name’s sake.
Luke describes what the "abomination of desolation" is:
Luke 21:20–22 (NKJV)
Destruction of Jerusalem [parallel passages
Matt. 24:15–21;
Mark 13:14–19]
20 “But when you see Jerusalem surrounded by armies, then know that its desolation is near. 21 Then let those who are in Judea flee to the mountains, let those who are in the midst of her depart, and let not those who are in the country enter her. 22 For these are the days of vengeance, that all things which are written may be fulfilled.
See that phrase "these are the days of vengeance"? This was part of Christ's earthly mission, as foretold by Isaiah:
Isaiah 61:1–2 (NKJV)
61 “The Spirit of the Lord GOD is upon Me,
Because the LORD has anointed Me
To preach good tidings to the poor;
He has sent Me to heal the brokenhearted,
To proclaim liberty to the captives,
And the opening of the prison to those who are bound;
2 To proclaim the acceptable year of the LORD,
And the day of vengeance of our God;
To comfort all who mourn,
As I wrote before, Revelation is, in part, a story of two tribulations: the tribulation of the saints, Christ's messengers, seen in chapters 1-8, and the judgment that comes on Jerusalem and Judea because of it in chapters 8-20. It is also a story, in part, of God divorcing the harlot and taking His new bride. It marks the end of one covenant (chapter 20) and the beginning of another (chapter 21).
Finally, I leave you with this: In Leviticus 26 and Deuteronomy 28, Moses tells Israel what the penalties for disobedience to the covenant would be. In Leviticus, he writes:
Leviticus 26:23–33 (NKJV)
23 ‘And if by these things you are not reformed by Me, but walk contrary to Me,
24 then I also will walk contrary to you, and I will punish you yet seven times for your sins.
25
And I will bring a sword against you that will execute the vengeance of the covenant;
when you are gathered together within your cities I will send pestilence among you;
and you shall be delivered into the hand of the enemy.
26 When I have cut off your supply of bread, ten women shall bake your bread in one oven, and they shall bring back your bread by weight, and you shall eat and not be satisfied.
27 ‘And after all this, if you do not obey Me, but walk contrary to Me,
28 then I also will walk contrary to you in fury;
and I, even I, will chastise you seven times for your sins.
29 You shall eat the flesh of your sons, and you shall eat the flesh of your daughters.
30 I will destroy your high places, cut down your incense altars, and cast your carcasses on the lifeless forms of your idols;
and My soul shall abhor you.
31 I will lay your cities waste and bring your sanctuaries to desolation, and I will not smell the fragrance of your sweet aromas.
32 I will bring the land to desolation, and your enemies who dwell in it shall be astonished at it.
33 I will scatter you among the nations and draw out a sword after you;
your land shall be desolate and your cities waste.
And what was the single, greatest act of rebellion against God that generation did?
Matthew 21:33–41 (NKJV)
33 “Hear another parable: There was a certain landowner who planted a vineyard and set a hedge around it, dug a winepress in it and built a tower. And he leased it to vinedressers and went into a far country. 34
Now when vintage-time drew near, he sent his servants to the vinedressers, that they might receive its fruit. 35 And the vinedressers took his servants, beat one, killed one, and stoned another. 36 Again he sent other servants, more than the first, and they did likewise to them. 37 Then last of all he sent his son to them, saying, ‘They will respect my son.’ 38 But when the vinedressers saw the son, they said among themselves, ‘This is the heir. Come, let us kill him and seize his inheritance.’ 39 So they took him and cast him out of the vineyard and killed him.
40 “Therefore, when the owner of the vineyard comes, what will he do to those vinedressers?”
41 They said to Him, “
He will destroy those wicked men miserably, and lease his vineyard to other vinedressers who will render to him the fruits in their seasons.”
They knew Christ was talking about them, and they killed him anyway. They did, indeed, fill up the measure of their guilt.
Understanding Revelation isn't hard. You just have to understand Christ's earthly mission.