Luke 21 is definitely in the past. When is the last time Jerusalem was compassed with armies? This speaks of the day of judgment of 70 AD. Jesus himself said "these are the days of vengeance", which can be tied to the souls under the altar asking for vengeance. And note that Jesus is speaking of a local event when he says "let them that are in Judea flee to the mountains" which is exactly what happened. All the Christians of Jerusalem fled into the mountains of Pela and thereby were spared the horrors of the time of judgment.
I think most Christians think this is future because they assume that when it speaks of Jesus coming in a cloud, they assume it is literal, but I understand this to be symbolic language of judgment. There are many such examples of God being "enrobed in clouds" when it describes God as being angry and meting out judgment.
Psalm 97:2
Clouds and darkness are round about him: righteousness and judgment are the habitation of his throne.
Jeremiah 4:13-14
Behold, he shall come up as clouds, and his chariots shall be as a whirlwind: his horses are swifter than eagles. Woe unto us! for we are spoiled. O Jerusalem, wash thine heart from wickedness , that thou mayest be saved. How long shall thy vain thoughts lodge within thee?
Nahum 1:3
The LORD is slow to anger, and great in power, and will not at all acquit the wicked: the LORD hath his way in the whirlwind and in the storm, and the clouds are the dust of his feet.
Zephaniah 1:15-17
That day is a day of wrath, a day of trouble and distress, a day of wasteness and desolation, a day of darkness and gloominess, a day of clouds and thick darkness, A day of the trumpet and alarm against the fenced cities, and against the high towers. And I will bring distress upon men, that they shall walk like blind men, because they have sinned against the LORD: and their blood shall be poured out as dust, and their flesh as the dung.
Revelation 10:1
Then I saw another mighty angel coming down from heaven. He was robed in a cloud, with a rainbow above his head; his face was like the sun, and his legs were like fiery pillars.
(Matthew 24 is the same)